A handful of functions unused functions were enabled during XIP builds,
which themselves didn't build correctly.  This just disables the
functions entirely.

Fixes: e8a62cc2 ("riscv: Implement sv48 support")
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20220420184056.7886-5-palmer@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>