pinctrl: pinconf-generic: Add properties 'skew-delay-{in,out}put-ps'

pinctrl: pinconf-generic: Add properties 'skew-delay-{in,out}put-ps'

Add the properties 'skew-delay-input-ps' and 'skew-delay-output-ps'
to the generic parameters used for parsing DT files. This allows to
specify the independent skew delay value for the two directions.
This enables drivers that use the generic pin configuration to get
the value passed through these new properties.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>