ARM: configs: keystone: add MTD_SPI_NOR (new dependency for M25P80)

This defconfig contains the CONFIG_M25P80 symbol, which is now
dependent on the MTD_SPI_NOR symbol. Add CONFIG_MTD_SPI_NOR to satisfy
the new dependency.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>