Documentation: DT: omap_serial: document missing compatible

The compatible "ti,am4372-uart" is used in arch/arm/boot/dts/am4372.dtsi
but not documented. Add necessary documentation.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>