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

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

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

The SPMI controller on Apple M2 Pro/Max/Ultra is compatible to
"apple,t8103-spmi" 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/

Reviewed-by: Neal Gompa <neal@gompa.dev>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Janne Grunau <j@jannau.net>