firmware: meson_sm: Rework driver as a proper platform driver

The secure monitor driver is currently a frankenstein driver which is
registered as a platform driver but its functionality goes through a
global struct accessed by the consumer drivers using exported helper
functions.

Try to tidy up the driver moving the firmware struct into the driver
data and make the consumer drivers referencing the secure-monitor using
a new property in the DT.

Currently only the nvmem driver is using this API so we can fix it in
the same commit.

Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>