sparc/leon3 irqmp: fix IRQ software ack

sparc/leon3 irqmp: fix IRQ software ack

With the LEON3 IRQ controller IRQs can be acknowledged 2 ways:
* Explicitly by software writing to the CLEAR_OFFSET register
* Implicitly when the procesor is done running the trap handler attached
  to the IRQ.

The actual IRQMP code only allows the implicit processor triggered IRQ ack.
If software write explicitly to the CLEAR_OFFSET register, this will clear
the pending bit in the register value but this will not lower the ongoing
raised IRQ with the processor. The IRQ will be kept raised to the LEON
processor until the related trap handler is run and the processor implicitly
ack the interrupt. So with the actual IRQMP code trap handler have to be run
even if the software has already done its job by clearing the pending bit.

This feature has been tested on another LEON3 simulator (tsim_leon3 from
Gaisler) and it turns out that the Qemu implementation is not equivalent to
the tsim one. In tsim, if software does clear a pending interrupt before
the related interrupt handler is triggered the said interrupt handler will
not be called.

This patch brings the Qemu IRQMP implementation in line with the tsim
implementation by allowing IRQ to be acknowledged by software only.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Reviewed-by: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>