mmc: sdhci-pxa: Add device tree support

Tested on an OLPC XO-1.75.  (MMP2, sdhci-pxav3, CONFIG_MACH_MMP2_DT=y)

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