xtensa: add default memmap and mmio32native options to defconfigs

Now that memory initialization doesn't add default memory region
specify it explicitly in the memmap command line option in case somebody
wants to boot in non-DT-enabled configuration.

While at it update earlycon access mode to mmio32native to support both
LE and BE cores transparently.

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