dt-bindings: spi: sunxi: Document new compatible strings

The Allwinner H6 SPI controller has advanced features over the H3
version, but remains compatible with it.
Document the usual "specific", "fallback" compatible string pair.
Also add the R40 version while at it.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>