configs: imx6: Don't define USDHC2_BASE_ADDR

USDHC base address will assigned by SPL using fsl_esdhc_initialize
and u-boot with devicetree, hence no remove base address assignment
in config files.

Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>