xtensa: mmu: fix page table initialization

xtensa_mmu_init() is called really early in the boot process
where the _kernel struct has not yet been initialized, and
thus we cannot use it to determine if the current CPU is
the boot CPU. In some cases, this may skip the call to
initialize the page tables which leaves us with incorrect
page table entries. Fix it by using a static variable to
determine whether the page tables have been initialized so
we only do it once per boot.

Fixes #76909

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