firmware: meson-sm: rework meson_sm_init to use module_platform_driver_probe

Commit 3aa0582f ("of: platform: populate /firmware/ node from
of_platform_default_populate_init()") takes care of populating
all the devices under the /firmware/ node in of_platform_default_populate_init()

This patch reworks meson_sm_init to use module_platform_driver_probe as
the platform device is populated.

Cc: Carlo Caione <carlo@caione.org>
Cc: Kevin Hilman <khilman@baylibre.com>
Cc: linux-amlogic@lists.infradead.org
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>