mmc: sunxi: Use mmc_of_parse()

At the moment the Allwinner MMC driver parses the bus-width and
non-removable DT properties itself, in the probe() routine.

There is actually a generic function provided by the MMC framework doing
this job, also it parses more generic properties like broken-cd and
advanced transfer modes.

Drop our own code and call mmc_of_parse() instead, to get all new
features for free.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>