fbdev: au1100fb: Fix build on MIPS64

fbdev: au1100fb: Fix build on MIPS64

Fix an error reported by the kernel test robot:
 au1100fb.c: error: implicit declaration of function 'KSEG1ADDR'; did you mean 'CKSEG1ADDR'?

arch/mips/include/asm/addrspace.h defines KSEG1ADDR only for 32 bit
configurations. So provide its compile-test stub also for 64bit mips builds.

Fixes: 6f366e86481a ("fbdev: au1100fb: Make driver compilable on non-mips platforms")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603042127.PT6LuKqi-lkp@intel.com/
Signed-off-by: Helge Deller <deller@gmx.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>