[ARM] pxa/zylonite: simplify reduntant gpio settings on mmc slot

PXA mmc host driver supports card detect, read only and power gpio pin
setting already. Zylonite platform driver needn't implement this any more.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>