dts: cadence_spi: Sync DT bindings with Linux

Adopt the Linux DT bindings and clean-up duplicate
and unused values.

Fix indentation of the QSPI node in the keystone k2g
device tree.

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>