doc: reference: drivers: update system drivers section

Update the system drivers section: remove references to
SYS_DEVICE_DEFINE, add references to the SYS_INIT() macro.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/reference/drivers/index.rst

@@ -382,21 +382,10 @@ function.
 System Drivers
 **************
 
-In some cases you may just need to run a function at boot. Special ``SYS_*``
-macros exist that map to ``DEVICE_DEFINE()`` calls.
-For ``SYS_INIT()`` there are no config or runtime data structures and there
-isn't a way
-to later get a device pointer by name. The same policies for initialization
-level and priority apply.
-
-For ``SYS_DEVICE_DEFINE()`` you can obtain pointers by name.
-
-:c:func:`SYS_INIT()`
-   Run an initialization function at boot at specified priority.
-
-:c:func:`SYS_DEVICE_DEFINE()`
-   Like :c:func:`DEVICE_DEFINE` without an API table and constructing
-   the device name from the init function name.
+In some cases you may just need to run a function at boot. For such cases, the
+:c:macro:`SYS_INIT` can be used. This macro does not take any config or runtime
+data structures and there isn't a way to later get a device pointer by name. The
+same device policies for initialization level and priority apply.
 
 Error handling
 **************