mmc: sdhci-pxav3: enhance device tree parser code

1. seperate device tree parsing from platform data handling which
   can make further work easy when platform data can be removed.

2. add calling mmc_of_parse which can parse more of property and
   pxav3_get_mmc_pdata can be shrinked a lot.

Signed-off-by: Kevin Liu <kliu5@marvell.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>