pinctrl: mediatek: Backward compatible to previous Mediatek's bias-pull usage

Refine mtk_pinconf_set()/mtk_pinconf_get() for backward compatibility to
previous MediaTek's bias-pull usage.
In PINCTRL_MTK that use pinctrl-mtk-common.c, bias-pull setting for pins
with 2 pull resistors can be specified as value for bias-pull-up and
bias-pull-down. For example:
    bias-pull-up = <MTK_PUPD_SET_R1R0_00>;
    bias-pull-up = <MTK_PUPD_SET_R1R0_01>;
    bias-pull-up = <MTK_PUPD_SET_R1R0_10>;
    bias-pull-up = <MTK_PUPD_SET_R1R0_11>;
    bias-pull-down = <MTK_PUPD_SET_R1R0_00>;
    bias-pull-down = <MTK_PUPD_SET_R1R0_01>;
    bias-pull-down = <MTK_PUPD_SET_R1R0_10>;
    bias-pull-down = <MTK_PUPD_SET_R1R0_11>;

On the other hand, PINCTRL_MTK_PARIS use customized properties
"mediatek,pull-up-adv" and "mediatek,pull-down-adv" to specify bias-pull
setting for pins with 2 pull resistors.
This introduce in-compatibility in device tree and increase porting
effort to MediaTek's customer that had already used PINCTRL_MTK version.
Besides, if customers are not aware of this change and still write devicetree
for PINCTRL_MTK version, they may encounter runtime failure with pinctrl and
spent time to debug.

This patch adds backward compatible to previous MediaTek's bias-pull usage
so that Mediatek's customer need not use a new devicetree property name.
The rationale is that: changing driver implementation had better leave
interface unchanged.

Signed-off-by: Light Hsieh <light.hsieh@mediatek.com>
Link: https://lore.kernel.org/r/1579675994-7001-5-git-send-email-light.hsieh@mediatek.com
Acked-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>