ARM: dts: OMAP3: disable RNG on N950/N9

Like on N900, we cannot access RNG directly on N950/N9. Mark it disabled in
the DTS to allow kernel to boot.

Fixes: 308607e5

 ("ARM: dts: Configure omap3 rng")
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>