devicetree: add accessors with default values

Add a variety of property and specifier cell accessors that fall back
on a default value supplied by the caller if the devicetree does not
contain the requested data.

This is useful for avoiding use of COND_CODE_1() and similarly tricky
macros in applications and drivers.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>