xtensa: disable MMU initialization option on MMUv2 cores

MMU initialization option is currently ignored on MMUv2 cores, but it is
used in Kconfig to select kernel load and start addresses. This choice
is not available for MMUv2 cores as they have hardwired TLB entries.
Disable MMU initialization option for known MMUv2 cores so that they get
correct kernel load/start address by default.
This fixes the default allmodconfig build.

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