ARM: dts: omap5.dtsi: add DSS RFBI node

The RFBI node for OMAP DSS was left out when adding the rest of the DSS
nodes, because it was not clear how to set up the clocks for the RFBI.

However, it seems that if there is a HWMOD for a device, we also need a
DT node for it. Otherwise, at boot, we get:

WARNING: CPU: 0 PID: 1 at arch/arm/mach-omap2/omap_hwmod.c:2542 _init+0x464/0x4e0()
omap_hwmod: dss_rfbi: doesn't have mpu register target base

Now that v3.17-rc3 contains a fix 8fd46439

 ("ARM: dts:
omap54xx-clocks: Fix the l3 and l4 clock rates") for the L3 ICLK
required by the RFBI, let's add the RFBI node to get rid of the
warning.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
[tony@atomide.com: updated description per comments from Nishant]
Signed-off-by: Tony Lindgren <tony@atomide.com>