arch/arm/mach-ixp4xx/include/mach/platform.h now gets included indirectly
and defines REG_OFFSET. Rename the register and bit definition to something
specific to the driver.

Fixes: 7fd70c65 ("ARM: irqstat: Get rid of duplicated declaration")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210710211431.1393589-1-alexandre.belloni@bootlin.com