mmc: esdhc-imx: parse max-frequency from devicetree

In order to make it possible to reduce the SD bus frequency,
parse the optional "max-frequency" attribute as documented in
devicetree/bindings/mmc/mmc.txt

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>