arm: add initial support for Amlogic P212 based on Meson GXL family

This adds platform code for the Amlogic P212 reference board based on a
Meson GXL (S905X) SoC with the Meson GXL configuration.

This initial submission only supports UART and MMC/SDCard, support for the
internal Ethernet PHY in Work In Progress.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Beniamino Galvani <b.galvani@gmail.com>