arm64: Support mapping a 52-bit physical adddress

When FEAT_LPA2 is enabled the physical address space increases from
48-bits to 52-bits. The top two address bits are moved to the now
unused shareability field.

Update the kernel to support this new larger address space.

Reviewed by: alc, kib
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46624