xtensa: initialize MMU before jumping to reset vector

When reset is simulated MMU need to be brought into its initial state,
because that's what bootloaders/OS kernels assume. This is especially
important for MMUv3 because TLB state when the kernel is running is
significatly different from its reset state.

With this change it is possible to boot linux and get back to U-Boot
repeatedly.

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