hw/arm_gic: Expose PPI inputs as gpio inputs

Expose the Private Peripheral Interrupt inputs as GPIO inputs.
The layout of the GPIO array is thus:
  [0..N-1] SPIs
  [N..N+31] PPIs for CPU 0
  [N+32..N+63] PPIs for CPU 1
  ...

Treating PPIs as being another kind of input line is in line with the
GIC architecture specification, where they are clearly described that
way. The 11MPCore TRM is a bit more ambiguous, but there is no practical
difference between "set PPI X as pending" and "0->1 transition on a
PPI input line configured as edge triggered", and PPIs are always
edge triggered, so this change won't affect behaviour.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
