xtensa: nommu: provide correct KIO addresses

KIO region location is different for noMMU cores. Provide different
default physical address and make KIO virtual address equal to physical.

Move xtensa_get_kio_paddr function close to XCHAL_KIO_PADDR definition
and define it not only for MMUv3, but for all MMU options except MMUv2.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>