keystone2: change default boot mode to ubi

To allow out of box demo, change default boot mode to ubi
boot now that NAND is functional in latest EVMs.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Reviewed-by: Tom Rini <trini@ti.com>