pinctrl: uniphier: refactor drive strength get/set functions

There is code duplication between uniphier_conf_pin_drive_get() and
uniphier_conf_pin_drive_set().  Factor out the common code into
uniphier_conf_get_drvctrl_data().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>