allwinner: H6: Factor out I2C platform setup

In the H6 platform code there is a routine to do the platform
initialisation of the R_I2C controller. We will need a very similar
setup routine to initialise the RSB controller on the A64.

Move this code to sunxi_common.c and generalise it to support all SoCs
and also to cover the related RSB bus.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
3 files changed