arm64: Make L1_BLOCKS_SUPPORTED dynamic

When FEAT_LPA2 is implemented and the TCR_EL1.DS field is set we can
create blocks at level 1 with a 16kB page size. Add a variable to
tell the kernel this is enabled.

This will be used by other parts of pmap.c with a 4k page kernel so
make it visible there.

Reviewed by: alc, kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46395