reset: sunxi: fix for 64-bit compilation

The Allwinner reset controller has 32-bit registers, so translating
the reset cell number into a register and bit offset should not use
any architecture dependent data size. Otherwise this breaks for 64-bit
architectures like arm64.
Fix this by making it clear that it's the hardware register width which
matters here in the calculation.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>