pinctrl: exynos: Extract pin parsing code into a separate function

Next commits are going to re-design the pin_to_bank_base() function and
its usage in a way that the pin parsing code will be called separately.
Extract it into a separate function first, as a refactoring commit.

No functional change.

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