imx: mx6slevk: Add support for USDHC1 and USDHC3 slots

There are three SD/MMC sockets on mx6slevk boards. Implements the
full support for them.
The default boot socket is USDHC2, so the MMC environment is set
to that device.

Signed-off-by: Ye.Li <B37916@freescale.com>