riscv/mm: Add XIP_FIXUP for riscv_pfn_base

This manifests as a crash early in boot on VexRiscv.

Signed-off-by: Myrtle Shah <gatecat@ds0.me>
[Palmer: split commit]
Fixes: 44c92257 ("RISC-V: enable XIP")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>