configs: rename ks2_evm into ti_armv7_keystone2

rename the keystone2 common header into an keystone2 architecture
specific header which can then reuse the common ti_armv7 config headers.

Acked-by: Vitaly Andrianov <vitalya@ti.com>
Acked-By: Murali Karicheri <m-karicheri2@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Nishanth Menon <nm@ti.com>