pinctrl: exynos: Rework pin_to_bank_base() to obtain data by name

Rework pin_to_bank_base() function to obtain bank data structure by bank
name instead of getting bank base address by pin name, and rename it to
get_bank() to reflect this change. This in turn leads to the extraction
of parse_pin(), so the caller has to use it before calling get_bank().

No functional change. This is a refactoring commit which prepares
pinctrl driver code for handling different sizes of register fields,
which will be added next.

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