riscv: dts: microchip: remove spi-max-frequency property

spi-max-frequency property is supposed to be a per SPI peripheral device
property, not a SPI controller property, so remove it.

Reported-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/lkml/20220526014141.2872567-1-robh@kernel.org/


Signed-off-by: Conor Dooley <conor.dooley@microchip.com>