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 <benoit.thebaudeau@advansee.com> Cc: Stefano Babic <sbabic@denx.de> Cc: Eric Bénard <eric@eukrea.com> Cc: Otavio Salvador <otavio@ossystems.com.br> Cc: Fabio Estevam <fabio.estevam@freescale.com> Cc: Jason Liu <r64343@freescale.com> Cc: Matt Sealey <matt@genesi-usa.com> Cc: Andy Fleming <afleming@gmail.com>