gpio: s3c: Fix the GPIO driver

The GPIO driver didn't correctly compute the bank offset
from the GPIO number and caused random writes into the
GPIO block address space. Fix the driver so it actually
does the writes correctly. While at it, make use of the
clrsetbits_le32() mechanisms.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>