powerpc: Correct parport interrupt parsing

Currently the parsing of the device tree in
arch/powerpc/include/asm/parport.h assumes that the interrupt provided in
the parallel port node is a valid virtual irq. The values for the
interrupts provided in the device tree should have meaning in the context
of the driver for the specific interrupt controller to which the interrupt
is connected and irq_of_parse_and_map() should be used to determine the
correct virtual irq.

Signed-off-by: Martyn Welch <martyn.welch@ge.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>