RISC-V: Fixmap support and MM cleanups

This patchset does:
1. Moves MM related code from kernel/setup.c to mm/init.c
2. Implements compile-time fixed mappings

Using fixed mappings, we get earlyprints even without SBI calls.

For example, we can now use kernel parameter
"earlycon=uart8250,mmio,0x10000000"
to get early prints on QEMU virt machine without using SBI calls.

The patchset is tested on QEMU virt machine.

Palmer: It looks like some of the code movement here conflicted with the
patches to move hartid handling around.  As far as I can tell the only
changed code was in smp_setup_processor_id(), and I've kept the one in
smp.c.