I2C unbusied also on GTA02

[Werner Almesberger]

GTA02 used to poll the PMU continuously when in the bootmenu. We know
from GTA01 that this causes conflicts with USB. I've now extended the
unbusy patch also for GTA02.

(Note: it doesn't really matter if we fail to clear all interrupts in
one sweep here. We'll get any stragglers in the next round.)

- Werner

----------------------------------- Changes -----------------------------------

Extended unbusying also to the GTA02.

unbusy-i2c.patch:
- board/neo1973/gta02/gta02.c (neo1973_new_second, neo1973_on_key_pressed): do
  not poll incessantly but only when the PMU has posted an interrupt
- board/neo1973/gta02/gta02.c (have_int, pwr_int_pending): detect when one of
  the interrupts we're looking for has occurred