sunxi: add gtbus-initialisation for sun9i

On sun9i, the GTBUS manages transaction priority and bandwidth
for multiple read ports when accessing DRAM. The initialisation
mirrors the settings from Allwinner's boot0 for now, even though
this may not be optimal for all applications (e.g. headless
systems might want to give priority to IO modules).

Adding a common callout to gtbus_init() from the SPL clock init
with a weakly defined implementation in sunxi/clock.c to fallback
to for platforms that don't require this.

[wens@csie.org: Moved gtbus_sun9i.c to arch/arm/mach-sunxi/; style cleanup]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>