x86: MMU: Enable boot time PAE page tables.

In PAE boot tables the __mmu_tables_start points to page directory
pointer (PDPT). Enable the PAE by updating the CR4.PAE and
IA32_EFER.NXE bits.

JIRA:ZEP-2511

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>