[ot] docs/opentitan: ot_eg_pad_ring.md: Add docs

Add initial pad ring documentation (only for the PoR signal, and how to
use it, for now).

Signed-off-by: Alex Jones <[email protected]>

Author: AlexJones0

docs/opentitan/ot_eg_pad_ring.md

@@ -0,0 +1,33 @@
+# OpenTitan Earl Grey Pad Ring
+
+## Power-on Reset (POR)
+
+The Earl Grey Pad Ring currently only supports the negated Power-on Reset (PoR) pad, which can
+be signalled by setting the `por_n` property and resetting the VM to perform a Power-on Reset.
+This can for example be done with the QEMU Monitor via the following command sequence:
+```
+> qom-set ot-eg-pad-ring.0 por_n low
+> system_reset
+> qom-set ot-eg-pad-ring.0 por_n high
+```
+
+Equivalent QMP JSON commands can also be used.
+
+Note that the current implementation directly invokes a reset request on any reset where a falling
+edge is detected (i.e. the reset strapping is asserted), and it is not well supported to "hold" the
+device in reset. If it is desired to emulate this time, you should stop and resume the VM for
+for the duration of the reset, e.g.:
+```
+/* Asserting the POR signal */
+> stop
+> qom-set ot-eg-pad-ring.0 por_n low
+> system_reset
+/* ... wait for the duration of the reset ... */
+/* De-asserting the POR signal */
+> qom-set ot-eg-pad-ring.0 por_n high
+> cont
+```
+
+## MIO Pads
+
+Currently, Earl Grey's MIO pads are not connected in the Pad Ring / Pinmux.