gpio: sunxi: Add compatible string for R40 PIO

The PIO on the R40 SoC is mostly compatible with the A20.
Only a few pin functions for mmc2 were added to the PC
pingroup, to support 8 bit eMMCs.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>