fsl_esdhc: Add device tree fixups

This patch implements fdt_fixup_esdhc() function that is used to fixup
the device tree.

The function adds status = "disabled" propery if esdhc pins muxed away,
otherwise it fixups clock-frequency for esdhc nodes.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Kim Phillips <kim.phillips@freescale.com>