riscv: Optimize kernel virtual address conversion macro

riscv: Optimize kernel virtual address conversion macro

The current test in kernel_mapping_va_to_pa only applies when
CONFIG_XIP_KERNEL is set, so use IS_ENABLED to optimize this macro at
compile-time in standard kernels that do not require this test.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Tested-by: Emil Renner Berthing <kernel@esmil.dk>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-By: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>