ARM: dts: imx6sl: fix rng node

rng DT node was added without a compatible string.

i.MX driver for RNGC (drivers/char/hw_random/imx-rngc.c) also claims
support for RNGB, and is currently used for i.MX25.

Let's use this driver also for RNGB block in i.MX6SL.

Fixes: e29fe21c ("ARM: dts: add device tree source for imx6sl SoC")
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>