keystone2: env: Set mmc as default boot for k2g-evm

For k2l, k2e and k2hk, ubi is set to default boot in uboot
environment settings; while for k2g, mmc should be the
default boot. This patch is to set mmc as default for k2g-evm

Signed-off-by: Yan Liu <yan-liu@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>