mmc: meson-axg: add support for the Meson-AXG platform

Introduce the compatible data to cover the register offset & mask
change of the eMMC controller in Amlogic's Meson-AXG SoC.

Signed-off-by: Nan Li <nan.li@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Kevin Hilman <khilman@baylibre.com>