RISC-V: Always define XIP_FIXUP

XIP depends on MMU, but XIP_FIXUP is used throughout the kernel in
order to avoid excessive ifdefs.  This just makes sure to always define
XIP_FIXUP, which will fix MMU=n builds.  XIP_OFFSET is used by assembly
but XIP_FIXUP is C-only, so they're split.

Fixes: 44c92257

 ("RISC-V: enable XIP")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Tested-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>