ppc: fix up trivial Kconfig config selection

Quoth BenH:
  "Ok, looks like I forgot to update the Kconfig for the new i2c driver,
   it should select I2C_POWERMAC instead.  Do you want a new patch or
   can you just fix it there ?"

Signed-off-by: Linus Torvalds <torvalds@osdl.org>