powerpc/warp: Platform fix for i2c change

A change to the i2c subsystem breaks the warp platform code. The patch
is cleaner anyway, the old way was a bit crufty.

For those with keen eyes, the gratuitous change in the string from
PIKA to Warp is just so the logs look a bit nicer. The following two
lines tend to be printed one after another.

  Warp POST OK
  Warp DTM thread running.

Yeah, this will be the third patch to warp.c submitted in this
release....

Cheers,
   Sean

The i2c_client struct changed, breaking the code that looked for the ad7414
chip. Use the new of_find_i2c_device_by_node function added in 2.6.29.

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