xtensa: extract init_kio

KIO region placement may be specified in the device tree, that's why
it's initialized with the rest of MMU after the early_init_devtree. In
order to support KASAN the MMU must be initialized earlier.
Separate KIO initialization from the rest of MMU initialization.
Reinitialize KIO if its location is specified in the device tree.

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