ARM: dts: omap2420-h4: Include omap2420.dtsi file instead the common omap2

Since the board is based on OMAP2420 we should include the dedicated dtsi
file (which includes the common omap2 dtsi).

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>