dts: cadence_spi: Update documentation for DT bindings

Update documentation to reflect adopting the Linux DT bindings.

Tested on TI K2G platform:
Tested-by: Vignesh R <vigneshr@ti.com>

Tested on a socfpga-cyclonev board:
Tested-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>

Signed-off-by: Jason Rush <jarush@gmail.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Acked-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Acked-by: Marek Vasut <marex@denx.de>