MLK-13440-2 mx6sllevk: Update USDHC2 parameter for using 1.8V I/O

Set the vs18_enable field to 1 for USDHC2 controller which connects to eMMC.
Also remove the explicit USDCH2 vendorspec register settings in board codes,
since the driver will take charge of it.

Signed-off-by: Ye Li <ye.li@nxp.com>