ARM: dts: Drop pointless status changing for am3 musb

The default is enabled, and there should be no need to reconfigure
the status for SoC internal devices in the board specific files.

Only the USB PHY used needs to be configured in the board specific
files.

Cc: Bin Liu <b-liu@ti.com>
Cc: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>