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 <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>
diff --git a/include/fsl_esdhc.h b/include/fsl_esdhc.h
index 4e321e7..47d2fe4 100644
--- a/include/fsl_esdhc.h
+++ b/include/fsl_esdhc.h
@@ -167,6 +167,7 @@
 
 struct fsl_esdhc_cfg {
 	u32	esdhc_base;
+	u32	sdhc_clk;
 };
 
 /* Select the correct accessors depending on endianess */