ARM: OMAP2+: omap2plus_defconfig: EHCI driver is not stable, disable it

The EHCI driver is not stable enough to be enabled by default.  In v3.5,
it has at least the following problems:

- warning dump during bootup
- hang during suspend
- prevents CORE powerdomain from entering retention during idle (even
  when no USB devices connected.)

This demonstrates that this driver has not been thoroughly tested and
therfore should not be enabled in the default defconfig.

In addition, the problems above cause new PM regressions which need be
addressed before this driver should be enabled in the default
defconfig.

Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>