Allow renames of derived peripherals

This commit enables two features (F):

F1. We're permitted to rename a peripheral that has a derivedFrom
attribute.
F2. When we rename a peripheral that has derivations, svd patch will
automatically update all derivations to use the new name.

These behaviors support renames of peripheral hierarchies. Suppose a
chip has two USB peripherals: USB_OTG1 and USB_OTG2. USB_OTG2 derives
from USB_OTG1. Before this commit, renaming these peripherals to USB1
and USB2 looks like this:

  _modify:  # Step 1: Rename the base peripheral USB_OTG1 => USB1.
    _peripherals:
      USB_OTG1:
        name: USB1
  _delete:  # Step 2: Delete USB_OTG2, because it can't be renamed.
    - USB_OTG2
  _add:     # Step 3: Add USB2, derived from USB1, with values from the SVD.
    USB2:
      derivedFrom: USB1
      baseAddress: 0x4042C000
      addressBlock: # Re-define address block
      interrupts:  # Re-define interrupts

Step 2 is required because modifications don't apply to derivedFrom
peripherals (line 180 in README.md). Step 3 means we duplicate SVD
information which was otherwise correct. This could be cumbersome when
trying to rename many instances.

(I wasn't able to figure out any other way to achieve this rename. Let
me know if this is already supported using other svdtool directives.)

F1 makes it easier to achieve the rename. There's no need for steps 2
and 3. When combined with F2, we write

  _modify:
    _peripherals:
      USB_OTG1:
        name: USB1
      USB_OTG2:
        name: USB2

F2 is a convenience. Without it, it's possible to use _derive directives
to fix the names. (You can't use _rebase though; _rebase needs both
peripherals to exist.) But _derive seems to focus on collapsing
independently-defined peripherals, which is not what we're doing when
changing peripheral names.

Author: mciantyre

svdtools/patch.py

@@ -435,6 +435,22 @@ def modify_peripheral(self, pspec, pmod):
         """Modify pspec inside device according to pmod."""
         for ptag in self.iter_peripherals(pspec):
             for (key, value) in pmod.items():
+                if key == "name":
+                    # If this peripheral has derivations, update the derived
+                    # peripherals to reference the new name.
+                    parent = self.device.find("peripherals")
+                    old_name = ptag.find("name").text
+                    for derived in parent.findall(
+                        "./peripheral[@derivedFrom='{}']".format(old_name)
+                    ):
+                        derived.set("derivedFrom", value)
+                    ptag.find("name").text = value
+                    continue
+
+                # We do not change derivedFrom peripherals beyond this point.
+                if "derivedFrom" in ptag.attrib:
+                    continue
+
                 if key == "addressBlock":
                     ab = ptag.find(key)
                     for (ab_key, ab_value) in value.items():