spi: dt-bindings: apple,spi: Add t6020-spi compatible

spi: dt-bindings: apple,spi: Add t6020-spi compatible

After discussion with the devicetree maintainers we agreed to not extend
lists with the generic compatible "apple,spi" anymore [1]. Use
"apple,t8103-spi" as base compatible as it is the SoC the driver and
bindings were written for.

The SPI controller on Apple M2 Pro/Max/Ultra SoCs is compatible with
"apple,t8103-spi" so add its per-SoC compatible with the former as
fallback used by the existing driver.

[1]: https://lore.kernel.org/asahi/12ab93b7-1fc2-4ce0-926e-c8141cfe81bf@kernel.org/

Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Janne Grunau <j@jannau.net>