yocto-bsp: re-enable AutoAddDevices in xorg.conf for generated BSPs

yocto-bsp: re-enable AutoAddDevices in xorg.conf for generated BSPs

This was added quite a long time ago because of poor interactions
between HAL and the X server when it came to enabling input devices.
HAL is long gone and I think it's safe to say we don't need to disable
this any longer, especially as it gets in the way of being able to plug
in the keyboard/mouse after boot.

(From meta-yocto rev: e06ab1e030e8cfbc259500b1a0b958fe752fb872)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>