ARM: dts: Drop custom hwmod property for am3 mmc

We can now probe devices with ti-sysc interconnect driver and dts
data. Let's drop the custom ti,hwmods dts property. We have already
dropped the platform data earlier, but have been still allocating it
dynamically, which is no longer needed.

Cc: Keerthy <j-keerthy@ti.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>