spi: spi-davinci: Update binding for 66AK2Gx pwr dm property

Add pm-domains property which is required for 66AK2Gx. Also document 66AK2G
unique clocks property usage.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>