tools: PCI: Add 'e' to clear IRQ

Add a new command line option 'e' to invoke "PCITEST_CLEAR_IRQ"
ioctl. This can be used to clear the irqs set using the 'i' option.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>