x86: mmu: reserve more space for page table if linking in virt

When linking in virtual address space, we still need physical
addresses in SRAM to be mapped so platform can boot from physical
memory and to access structure necessary for boot (e.g. GDT and
IDT). So we need to enlarge the reserved space for page table
to accommodate this.

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