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>