[POWERPC] Fix i2c-powermac platform device usage

i2c-powermac was written & merged right after Russell King's changes
adding platform_driver... which I missed. Thus it still used struct
device, causing crashes when hitting sleep/wakeup callbacks (it happened
to work by luck so far, until early/late callbacks got added). This
causes crashes on sleep/wakeup on PowerBooks with 2.6.19. The patch
fixes it by using a proper platform_driver.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>