ARM: dts: Configure omap3 rng

Looks like omap3 RNG is similar to the omap2 rng, let's get it working
by configring the dts node for it.

We must also add rng_ick to core_l4_clkdm as noted by Adam Ford.

And please note that the RNG is likely disabled on HS devices. At least
n900 does not have it accessible, and instead omap3-rom-rng driver must
be used. So let's tag RNG as disabled on n900 as noted by Pali Rohár
<pali.rohar@gmail.com>.

On am3517 at least the clocks need to be configured to get it working
as noted by Adam Ford, so let's tag it disabled for now.

Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Adam Ford <aford173@gmail.com>
Cc: Pali Rohár <pali.rohar@gmail.com>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: Tero Kristo <t-kristo@ti.com>
Tested-by: Adam Ford <aford173@gmail.com> #logicpd-torpedo-37xx-devkit
Signed-off-by: Tony Lindgren <tony@atomide.com>