riscv: mm: init: try IS_ENABLED(CONFIG_XIP_KERNEL) instead of #ifdef

Try our best to replace the conditional compilation using
"#ifdef CONFIG_XIP_KERNEL" with "IS_ENABLED(CONFIG_XIP_KERNEL)", to
simplify the code and to increase compile coverage.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>