riscv: Introduce va_kernel_pa_offset for 32-bit kernel

va_kernel_pa_offset was only used for 64-bit as the kernel mapping lies
in the linear mapping for 32-bit kernel and then only the offset between
the PAGE_OFFSET and the kernel load address is needed.

But this distinction complexifies the code with #ifdefs and especially
with a separate definition of the address conversions macros.

Simplify the code by defining this variable for both 32-bit and 64-bit.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>