aspeed/soc: Introduce a new API to get the device irq

aspeed/soc: Introduce a new API to get the device irq

Currently, users can set the INTC mapping table with
enumerated device id and device irq to get the INTC orgate
input pins. However, some devices use the continuous source numbers in the
same INTC orgate. To reduce the enumerated device id definition,
create a new API to get the INTC orgate input pin
if users only provide the device id with its bus number index.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>