firmware: meson_sm: Add serial number sysfs entry

The Amlogic Meson SoC Secure Monitor implements a call to retrieve an unique
SoC ID starting from the GX Family and all new families.

The serial number is simply exposed as a sysfs entry under the firmware
sysfs directory.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>