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>
diff --git a/plat/allwinner/common/include/sunxi_private.h b/plat/allwinner/common/include/sunxi_private.h
index 54cc27e..6c9c509 100644
--- a/plat/allwinner/common/include/sunxi_private.h
+++ b/plat/allwinner/common/include/sunxi_private.h
@@ -19,5 +19,6 @@
 
 uint16_t sunxi_read_soc_id(void);
 void sunxi_set_gpio_out(char port, int pin, bool level_high);
+int sunxi_init_platform_r_twi(uint16_t socid, bool use_rsb);
 
 #endif /* SUNXI_PRIVATE_H */