env: kconfig: Restrict rockchip env for MMC

Rockchip do support SPI flash as well, so there is
a possibility of using flash environment for those
use cases.

So, restrict the current env offset, size for MMC.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>