arm64: Enable FEAT_E0PD when supported

FEAT_E0PD adds two fields to the tcr_el1 special register that, when
set, cause userspace access to either the top or bottom half of the
address spaces without a page walk.

This can be used to stop userspace probing the kernel address space
as the CPU will raise an exception in the same time if the probed
address is in the TLB or not.

Reviewed by: kevans
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D41760