sunxi: Introduce common symbol for H6 like SoCs

It turns out that there are at least 2 other SoCs which have basically
the same memory map, similar clocks and other features as H6. It's very
likely that we'll see more such SoCs in the future. In order to ease
porting to new SoCs and lower ifdef clutter, introduce common symbol for
them.

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>