riscv: mm: init: remove _pt_ops and use pt_ops directly

Except "pt_ops", other global vars when CONFIG_XIP_KERNEL=y is defined
as below:

|foo_type foo;
|#ifdef CONFIG_XIP_KERNEL
|#define foo	(*(foo_type *)XIP_FIXUP(&foo))
|#endif

Follow the same way for pt_ops to unify the style and to simplify code.

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