pinctrl: exynos: Support different register types in pin banks

Get rid of hard-coded register offsets and widths. Instead provide a way
for pinctrl drivers to specify different pin bank register offsets and
widths. This in turn makes it possible to add support for new SoCs that
have registers with offset/width values different than generic ones
already available in pinctrl-exynos driver.

Offset constants (now unused in pinctrl-exynos.c) are moved to
pinctrl-exynos7420 driver, which is the single user of those constants.

The design of this patch follows Linux kernel pinctrl-exynos driver
design, in terms of added data structures and types. This patch doesn't
add support for any new SoCs and shouldn't introduce any functional
changes.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>