a10_gpio_get_function now returns the whole function not only
GPIO_INPUT/GPIO_OUTPUT.
a10_gpio_get_pud now returns the whole pud not only PULLDOWN/PULLUP.
Add a10_gpio_get_drv to get the current drive strenght.
During fdt pin configure, avoid setting function/drive/pud if it's already in
the correct value.

Tested on Allwinner H3 and A20

MFC after: 1 week