configs:ks2_evm: update defconfigs to support SPL

The K2HK and K2E boards support SPL by default, so add
CONFIG_SPL option. Also export CONFIG_ARM, CONFIG_ARCH_KEYSTONE
and TARGET_K2*_EVM options to spl/.config as they are the same.
So now it's convinient to build gph images using only two commands:

make k2hk_evm_defconfig
make u-boot-spi.gph

Acked-By: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>