Add definitions for PCIe legacy INTx interrupts.

This is required for example in a scenario where a driver requests only
one of the legacy interrupts (INTA). Without this, the driver would be
notified on events on all 4 (INTA, INTB, INTC, INTD), even if it
requested only one of them.
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>