mach-sunxi: Add SPL SPI boot for SUNIV

The SUNIV SoCs come with a sun6i-style SPI controller at the base address
of sun4i SPI controller. The module clock of the SPI controller is
missing which leaves us running directly from the AHB clock, which is
set to 200MHz.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
[Icenowy: Original implementation]
Signed-off-by: Jesse Taube <Mr.Bossman075@gmail.com>
[Jesse: adaptation to Upstream U-Boot]
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>