stand/efi: Limit where we load the arm64 kernel

When FEAT_LPA2 is implemented the hardware supports increasing the
physical address space from 48-bit to 52-bit.

As older kernels only support a 48-bit physical address space limit
where the kernel is loaded to this.

Reviewed by: kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46625