ARM: bcm2835: dt: Add header file for pinctrl constants

This new header file defines pincontrol constants to use
from bcm2835 DTS files for pincontrol properties option.

Reviewed-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>