powerpc: Update Warp to use leds-gpio driver

Now that leds-gpio is a proper OF platform driver, the Warp can use
the leds-gpio driver rather than the old out-of-kernel driver.

One side-effect is the leds-gpio driver always turns the leds off
while the old driver left them alone. So we have to set them back to
the correct settings.

Signed-off-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>