MAINTAINERS: Update entry for omap related .dts files to cover new SoCs

DRA7(including AM5x) and AM47x series are handled under OMAP umbrella.
These SoC support and dts have been added since 3.14 kernel and Pull
requests for these have come in from OMAP till date.

So just ensure that get_maintainers can pick up this list as well.

Cc: linux-omap@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Benoît Cousson <bcousson@baylibre.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>