riscv/mm: Add XIP_FIXUP for phys_ram_base

This manifests as a crash early in boot on VexRiscv.

Signed-off-by: Myrtle Shah <gatecat@ds0.me>
[Palmer: split commit]
Fixes: 6d7f91d9

 ("riscv: Get rid of CONFIG_PHYS_RAM_BASE in kernel physical address conversion")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>