spi: migrate mt7621 text bindings to YAML

SoC MT7621 SPI bindings used text format, so migrate them to YAML.
There are some additions to the binding that were not in the original
file. This binding is used in MT7621 and MT7628a Ralink SoCs. To
properly match both dts nodes in tree we need to add to the schema
'clocks', 'clock-names' and 'reset-names'. Both 'clock-names' and
'reset-names' use 'spi' as string so maintain that as const in
the schema.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20220927031929.807070-1-sergio.paracuellos@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>