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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>