pinctrl: pinctrl-nomadik: Append sleepmode property with vendor specific prefixes

Any non-standard property should contain the vendor's identifier which
should be perpended onto the property name followed by a comma. This
aids in name-space collision prevention. This patch ensures the
sleepmode property adheres to the rules.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>