1. Add nodes for the two SPI controllers found in MIPS Creator CI20.
2. Reparent SPI clock source to effectively use MPLL and set its clock
   rate to 54MHz.

NOTE: To use the SPI controllers, `pinctrl-0` property must be set in
order to configure the used pins. As SPI functionality is multiplexed on
multiple pin groups, this choice is left to the user.

An example configuration:
```
 &spi0 {
         pinctrl-0 = <&pins_spi0>;
 }

 pins_spi0: spi0 {
         function = "ssi0";
         groups = "ssi0-dt-e", "ssi0-dr-e", "ssi0-clk-e",
                  "ssi0-ce0-e", "ssi0-ce1-e";
         bias-disable;
 };
```
Consult the CI20 pinout description for more details.
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Link: https://lore.kernel.org/r/20210830230139.21476-4-contact@artur-rojek.euSigned-off-by: Mark Brown <broonie@kernel.org>