mmc: add new sdhci-pxa driver for Marvell SoCs

Support Marvell PXA168/PXA910/MMP2 SD Host Controller.

Signed-off-by: Zhangfei Gao <zhangfei.gao@marvell.com>
Acked-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Chris Ball <cjb@laptop.org>