arm: meson: add sm cmd to retrieve SoC serial

The Secure Monitor offers multiple services, like returning the
SoC unique serial number, already used to generate an unique MAC
address.

This adds a new, Amlogic specific, "sm" cmd with a "serial" subcommand
to write the SoC unique serial to memory.

This "cm" command will be extended in further patches.

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