media: ipu3-cio2: Build only for x86

According to the original code in the driver it was never assumed to work
with big page sizes: unsigned short type followed by PAGE_SHIFT and
PAGE_MASK which may be different on non-x86 architectures.

Recently LKP found an issue on non-x86 architectures due to above
mentioned limitations. Since Sakari acknowledges that it's not really
useful to be able to compile this elsewhere, mark it x86 only.

Fixes: a31d19f8

 ("media: ipu3: allow building it with COMPILE_TEST on non-x86 archs")
Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>