ARM: dts: meson-sm1: add u-boot specific MMC controller compatible

In order to enable the Amlogic SM1 MMC controller fix, we need to add a u-boot specific
MMC controller compatible.

This adds a new meson-sm1-u-boot.dtsi and reworks the other -u-boot.dtsi
to use this for SM1 based boards.

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