ARM: dts: i.MX25: add RNGB node to dtsi

Add a devicetree entry for the Random Number Generator Version B (RNGB).
The driver for RNGC supports version B as well.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>