mxc: Fix SDHC multi-instance clock

mxc: Fix SDHC multi-instance clock

On mxc, each SDHC instance has a dedicated clock, so gd->sdhc_clk is not
suitable for the multi-instance use case (initialization made directly with
fsl_esdhc_initialize()).

This patch fixes this issue by adding a configuration field for the SDHC input
clock frequency.

Signed-off-by: Benoît Thébaudeau <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Eric Bénard <[email protected]>
Cc: Otavio Salvador <[email protected]>
Cc: Fabio Estevam <[email protected]>
Cc: Jason Liu <[email protected]>
Cc: Matt Sealey <[email protected]>
Cc: Andy Fleming <[email protected]>