xtensa: mmu: handle TLB misses in C exception handler

This moves the TLB miss handling to the C exception handler.
This also allows us to handle page faults (for example,
unmapped pages) during this time as any more exceptions
handled in the C handler will not trigger the double
exception handler but the same C handler.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>