ARM: dts: sun7i: Add can0_pins_a pinctrl settings

The A20 SoC has an on-board CAN controller. This patch adds
the pinctrl settings for pins PH20 and PH21.

This patch is adapted from the description in
Documentation/devicetree/bindings/net/can/sun4i_can.txt

Signed-off-by: Patrick Menschel <menschel.p@posteo.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>