arm64: Add the tcr_el2 ds field

This will be used to support FEAT_LPA2 to allow more than 48 bits of
physical address space.

Reviewed by: alc, kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46392

(cherry picked from commit 7be11454edd4eca6cbd299cb5b486294f912ffbd)