reset: intel: fix a compile warning about REG_OFFSET redefined

kernel test robot reports a compile warning about REG_OFFSET redefined
in the reset-intel-gw.c after merging commit e44ab4e1 ("regmap:
Simplify implementation of the regmap_read_poll_timeout() macro"). the
warning is like that:

drivers/reset/reset-intel-gw.c:18:0: warning: "REG_OFFSET" redefined
 #define REG_OFFSET GENMASK(31, 16)

In file included from ./arch/arm/mach-ixp4xx/include/mach/hardware.h:30:0,
                 from ./arch/arm/mach-ixp4xx/include/mach/io.h:15,
                 from ./arch/arm/include/asm/io.h:198,
                 from ./include/linux/io.h:13,
                 from ./include/linux/iopoll.h:14,
                 from ./include/linux/regmap.h:20,
                 from drivers/reset/reset-intel-gw.c:12:
./arch/arm/mach-ixp4xx/include/mach/platform.h:25:0: note: this is the location of the previous definition
 #define REG_OFFSET 3

Reported-by: kernel test robot <lkp@intel.com>
Fixes: c9aef213 ("reset: intel: Add system reset controller driver")
Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>