mmc: mvsdio: use standard MMC device-tree binding parser mmc_of_parse()

Instead of parsing the DT binding on our own, use the standard parser
mmc_of_parse(), introduced by commit 6c56e7a0.

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Chris Ball <cjb@laptop.org>