ARM: ti: consolidate mmc environment variables

Introduce include/environment/ti/mmc.h that
consolidates environment variable definitions
for various TI boards that support MMC/SD.

This allows reuse of same environment variables
on non-ARMv7 TI platforms like OMAP-L138 for
example.

While at it, move DFU-related environment variable
includes to only non-SPL builds for AM335x and
AM437x since they are not really used for SPL
today.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>