mx35: Fix eSDHC clocks

Each eSDHC instance has a dedicated clock.

gd->sdhc_clk must also 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>