fsl_esdhc: Fix esdhc disabled problem on some platforms

Some new platform's esdhc pins don't share with other function.
The eSDHC shouldn't be disabled, even if "esdhc" isn't defined
in hwconfig env variable.

Use CONFIG_FSL_ESDHC_PIN_MUX to fix this problem.

Signed-off-by: Chenhui Zhao <b26998@freescale.com>
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>