riscv: Simplify xip and !xip kernel address conversion macros

To simplify the kernel address conversion code, make the same definition of
kernel_mapping_pa_to_va and kernel_mapping_va_to_pa compatible for both xip
and !xip kernel by defining XIP_OFFSET to 0 in !xip kernel.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>