mx25: Fix eSDHC support

The MMC driver appropriate for the i.MX25 is fsl_esdhc, which has nothing to do
with mxcmmc.

Also, each eSDHC 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>