[ARM] 5583/1: VIC: acknowledge software interrupts

The PrimeCell Vectored Interrupt Controller offers a way to trigger
any interrupt through software. This is a useful tool for developing,
but such software interrupt can only be acked by writing a bit in the
"software clear" register, or the handler will loop forever.  This
splits ack from mask, and acks the soft irq in case it was the source.

Signed-off-by: Alessandro Rubini <rubini@unipv.it>
Acked-by: Andrea Gallo <andrea.gallo@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>