mx5/6 clocks: Fix SDHC clocks

The i.MX5 eSDHC clocks were considered as coming from the IPG clock although
they have dedicated clock paths.

Also, on i.MX5/6, each SDHC instance has a dedicated clock, so gd->sdhc_clk must
be set accordingly. This is good for the case only a single SDHC instance is
used (initialization made with fsl_esdhc_mmc_init()). A future patch will fix
the multi-instance use case (initialization made directly with
fsl_esdhc_initialize()).

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>