arm: meson: add sm command to retrieve the reboot reason

The Secure Monitor offers multiple services, like returning the
SoC unique serial number, and can provide the "reboot reason" as
set by the previous booted system.

This extends the Amlogic specific "sm" cmd with a "reboot_reason" subcommand
to print or set a specified environment variable with the reboot reason in
human readable format.

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