mmc: dt-bindings: update bindings doc to support Meson-AXG SoC

Update the documentation to list support for Meson-AXG SoC explicitly.
The new binding string is necessary since this SoC introduce a few
IP difference comparing to previous old generation.

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>