ABI: sysfs-bus-platform: add modalias description

Define the modalias parameter for platform devices, including
the ones exposed via devicetree.

Acked-by: Barry Song <song.bao.hua@hisilicon.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9cdebbad008886b1d09f5f3ac5d88bee19f08d97.1632750608.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-bus-platform

@@ -42,3 +42,15 @@ Date:		August 2021
 Contact:	Barry Song <song.bao.hua@hisilicon.com>
 Description:
 		This attribute will show "msi" if <N> is a valid msi irq
+
+What:		/sys/bus/platform/devices/.../modalias
+Description:
+		Same as MODALIAS in the uevent at device creation.
+
+		A platform device that it is exposed via devicetree uses:
+
+			- of:N`of node name`T`type`
+
+		Other platform devices use, instead:
+
+			- platform:`driver name`