musicpal: Rework GPIO input events

musicpal: Rework GPIO input events

The qdev_gpio conversion of 343ec8e caused come polarity mismatch of key
event pins and left an overly complex solution behind. Take this chance
and refactor the GPIO input system of the Musicpal, moving it closer to
reality:
 - Instantiate all 32 GPIO input pins and do the routing only via
   qdev_connect_gpio_out.
 - Implement IMR and IER registers. They manage the GPIO pin IRQ. IMR
   seems to enable IRQs on rising edges, IER on falling ones. At least
   this matches what the Musicpal fireware require.
 - Move key pin logic inversion from the GPIO layer to musicpal_key.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>