The Allwinner H6 SoC contains two SPI controllers similar to the H3/A64,
but with the added capability of 3-wire and 4-wire operation modes.
For now the driver does not support those, but the SPI registers are
fully backwards-compatible, just adding bits and registers which were
formerly reserved. So we can use the existing driver in "legacy" SPI
modes, for instance to access the SPI NOR flash soldered on the PineH64
board.
We use an H6 specific compatible string in addition to the existing H3
string, so when the driver later gains QSPI support, it should work
automatically without any DT changes.

Tested by accessing the SPI flash on a Pine H64 board (SPI0), also
connecting another SPI flash to the SPI1 header pins.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>