davinci: DA830/OMAP-L137 EVM: do not configure NAND on UI card when MMC/SD is selected

On the DA830, AEMIF and MMC/SD pins are shared. On the EVM, when
the mux_mode signal is low MMC/SD works and when mux_mode signal
is high, NAND works.

When MMC/SD driver is configured in the kernel, do not let NAND
get registered and drive mux_mode high. Instead, print a warning
for user to understand why the platform device for NAND did not
get registered.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>