sunxi: prcm: Add memory map for H6 like SoCs

There was no need to have prcm definitions for H6 and similar SoCs till
now. However, support R_I2C will be needed soon in SPL.

Move old definitions to prcm_sun6i.h and add new ones in prcm_sun50i.h.
One of those files will be selected in common prcm.h based on defined
macros.

This commit doesn't do any functional change.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>