xtensa: mmu: clear ZSR_DEPC_SAVE at boot

ZSR_DEPC_SAVE is being used to determine whether we are faulting
inside double exception if this is not zero. It is possible that
the boot ROM or custom startup code leaves this non-zero, which
would result in a fake triple fault. So clear it at boot. Note
that the zeroing is done in MMU init code as these triple
faults are not actual hardware ones but only semantics, and will
occur once MMU is enabled.

Fixes #75194

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