spi: meson-spicc: add min sclk for each compatible

The G12A SPICC controller variant takes the source clock from a specific
clock instead of the bus clock.
The minimal clock calculus won't work with the G12A support, thus add the
minimal supported clock for each variant and pass this to the SPI core.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20200312133131.26430-6-narmstrong@baylibre.com


Signed-off-by: Mark Brown <broonie@kernel.org>