ARM: dts: am4372: let boards access all nodes through labels

By providing labels for rtc, wdt, cpu and dispc nodes,
boards can access them to add board-specific data.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Tested-by: Franklin Cooper Jr. <fcooper@ti.com>
Tested-by: Tom Rini <trini@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>