parisc: Use absolute_pointer() to define PAGE0

Use absolute_pointer() wrapper for PAGE0 to avoid this compiler warning:

  arch/parisc/kernel/setup.c: In function 'start_parisc':
  error: '__builtin_memcmp_eq' specified bound 8 exceeds source size 0

Signed-off-by: Helge Deller <deller@gmx.de>
Co-Developed-by: Guenter Roeck <linux@roeck-us.net>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>