ARM: dts: am437x-l4: Drop ti,omap2-uart entry from UART nodes

ti,omap2-uart was kept around to work with legacy omap-serial driver.
Now that we have completed move to 8250-omap.c drop legacy compatible.
This will simplify writing YAML schema.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>