KVM: selftests: arm64: Fix ttbr0_el1 encode for PA bits > 48

Bits [51:48] of the pgd address are stored at bits [5:1] of ttbr0_el1.
page_table_test stores its page tables at the far end of IPA space so
was tripping over this when run on a system that supports FEAT_LPA (or
FEAT_LPA2).

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>