arm64: lop off another 24MB of KVA for early device mappings

This grows the block enough to fit a 4K 32-bit depth framebuffer; some
firmware would present smaller GOP modes to be able to boot with a
smaller framebuffer on these devices, but the Windows Devkit firmware
is simply not that nice. Instead, it offers exactly one GOP mode that
matches the current resolution of the attached display, so with limited
control over resolution on most of my displays it'd be nice if we could
Just Work(TM) at 4K.

andrew notes that he has some ideas for removing PMAP_MAPDEV_EARLY_SIZE
entirely, so this limitation could end up removed altogether in the
future.

Reviewed by: andrew, emaste
Differential Revision: https://reviews.freebsd.org/D42726