esdhc: Detecting 8 bit width before mmc initialization

The upper 4 data signals of esdhc are shared with spi flash.
So detect if the upper 4 pins are assigned to esdhc before
enable sdhc 8 bit width.

Signed-off-by: Haijun Zhang <haijun.zhang@freescale.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Reviewed-by: York Sun <yorksun@freescale.com>