gpio: amdpt: Convert to use gpio-generic

Use gpio-generic to simplify this driver.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Tested-by: YD Tseng <Yd_Tseng@asmedia.com.tw>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>