[ARM] 4589/1: ns9xxx: acknowledge IRQ_EXT2 in the demux routine for FPGA irqs

The FPGA on the a9m9750dev board interrupts the CPU via EXT2. So
to acknowledge any FPGA interrupt IRQ_EXT2 must be acknowledged.

Signed-off-by: Uwe Kleine-König <ukleinek@informatik.uni-freiburg.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>