mmc: meson: update doc to support Meson-AXG platform

Explicitly update the docomentation to support the Meson-AXG platform.

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>