ARM: dts: Drop legacy custom hwmods property for dra7 gpio

With recent ti-sysc driver changes, we can probe most devices with device
tree data only and drop the custom "ti,hwmods" property.

We have already added the related device tree data earlier, and have
already dropped the platform data. But we have been still dynamically
allocating the platform data based on "ti,hwmods" property. With recent
ti-sysc driver changes this is no longer needed.

Signed-off-by: Tony Lindgren <tony@atomide.com>