powerpc/4xx: Have Warp take advantage of GPIO LEDs default-state = keep

The GPIO LEDS driver now has a default state of "keep".  Update the Warp DTS
and platform file to take advantage of this new state.  This removes the
hardcoding of the two LEDs on the Warp.

Signed-off-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>