arm64: dts: sun50i-a64: Add i2c2 pins

PinePhone needs I2C2 pins description. Add it, and make it default
for i2c2, since it's the only possiblilty.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>