Handle vqmmc supply. Some boards have a fixed I/O voltage
at 1.8V for emmc, so the usdhc also needs to be configured
as 1.8V by setting VSELECT bit. The vs18_enable is the one
that used to checking whether setting VSELECT or not in
the driver. So if vqmmc supply is 1.8V, set vs18_enable,
the driver will set VSELECT.
Signed-off-by: Peng 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> Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
