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: NLucas Stach <l.stach@pengutronix.de>
Acked-by: NShawn Guo <shawn.guo@linaro.org>
Signed-off-by: NChris Ball <cjb@laptop.org>