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

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

Let's drop it for omap4 MMC as we can test that for runtime PM
for core retention idle mode for wlcore WLAN.

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>