arm64: Fix typo in add_default_hugepagesz() for 64K pages

The default hugepage size when 64K pages are enabled is set to 2MB using
the contiguous PTE bit. The add_default_hugepagesz(), however, uses
CONT_PMD_SHIFT instead of CONT_PTE_SHIFT. There is no functional change
since the values are the same.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>