When using eMMC with 1.8V I/O, the VSELECT bit need to be set in
the USDHC controller when init.

This patch adds a parameter "vs18_enable" in fsl_esdhc_cfg
structure and priv data, so each controller can have different
settings.

We could not use CONFIG_SYS_FSL_ESDHC_FORCE_VSELECT, it has problem
that it will apply to all USDHC controllers and it only set the 1.8V
at init phase. So if user does not select to the eMMC device,
the voltage on the I/O pins are not correct.
Signed-off-by: NPeng Fan <peng.fan@nxp.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: York Sun <york.sun@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>