ARM: dts: suniv: F1C100: add SPI support

The F1C100 series contains two SPI controllers, and many boards use SPI0
for a SPI flash, as the BROM is able to boot from that.

Describe the two controllers in the SoC .dtsi, and also add the PortC
pins for SPI0, since this is where BROM looks at when trying to boot
from the commonly used SPI flash.

The SPI controller seems to be the same as in the H3 chips, but it lacks
a separate mod clock. The manual says it's connected to AHB directly.
We don't export that AHB clock directly, but can use the AHB *gate* clock
as a clock source, since the SPI driver is not supposed to change the AHB
frequency anyway.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20220317162349.739636-12-andre.przywara@arm.com