ARM: dts: DRA7: wdt: Fix compatible property for watchdog node

OMAP wdt driver supports only ti,omap3-wdt compatible. In DRA7 dt
wdt compatible property is defined as ti,omap4-wdt by mistake instead of
ti,omap3-wdt. Correcting the typo.

Fixes: 6e58b8f1 ("ARM: dts: DRA7: Add the dts files for dra7 SoC and dra7-evm board")
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>