mmc: omap_hsmmc: support more DT properties

This should probably be done implicitly through mmc_of_parse(), but that
doesn't play well along with the multi-slot model the hsmmc driver
features. Hence, for now, do it manually. The properties are already
documented in Documentation/devicetree/bindings/mmc/mmc.txt.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <chris@printf.net>