ARM: dts: Drop legacy custom hwmods property for omap4 uart

With recent ti-sysc driver changes, we can now finally probe most
modules without needing the custom ti,hwmods property.

Let's start with omap4 uart as we can test that for runtime PM
for core retention idle mode.

Cc: devicetree@vger.kernel.org
Cc: Rob Herring <robh@kernel.org>
Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>