dt-bindings: pincfg-node: Add properties 'skew-delay-{in,out}put-ps'

dt-bindings: pincfg-node: Add properties 'skew-delay-{in,out}put-ps'

Add the properties 'skew-delay-input-ps' and 'skew-delay-output-ps'
to specify independent skew delay value for the two pin's directions.
Make the new properties unavailable when the existing property
'skew-delay' is selected.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>