arm64: coresight: fix the build without FDT

coresight_cpu_debug only has an FDT attachment, so let's not build it
for kernels without FDT.

coresight.h includes sys/malloc.h via header pollution
dev/ofw/openfirm.h; include it directly in case we're building without
FDT.

Reviewed by: andrew, manu
Differential Revision: https://reviews.freebsd.org/D36789