pxamci: introduce mmc_has_26mhz() and include pxa935

Along with more processor supporting 26MHz mode (including pxa935),
introduce an individual macro mmc_has_26mhz() for this.

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