handle /reserved-memory nodes from device trees on arm64.

handle /reserved-memory nodes from device trees on arm64.

u-boot is supposed to take these entries and put them in the efi
memory map, but i keep hitting machines where an otherwise functional
u-boot does not do this, resulting in weird errors.

i have an espressobin with a vendor u-boot that has a reserved-memory
region for psci. without this diff the machine faults when the
kernel tries to reboot using a psci handler.

a macchiatobin with an otherwise working u-boot throws SErrors or
panics on weird memory corruption problems without this. i thought
it was bad RAM, but the problems persisted with completely different
ram, and very underclocked and well cooled ram.

riscv64 already has code to handle reserved-memory regions. the
riscv64 change is to add handling for the "no-map" property.

ok kettenis@