media: ipu2-cio2: Replace custom definition with PAGE_SIZE

It's quite unlikely that another page size will be supported,
but in any case there is still an inconsistency between custom
page size definition and generic macros used in the driver.

Switch over to the generic PAGE_SIZE for sake of the consistency.

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>