ARM: dts: armada-370-rn104: add pinmuxing for i2c0

Up to now a working i2c bus depended on the bootloader to configure the
pinmuxing. Make it explicit.

As a side effect this change makes i2c work in barebox.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>