mailbox: zynqmp: handle SGI for shared IPI

At least one IPI is used in TF-A for communication with PMC firmware.
If this IPI needs to be used by other agents such as RPU then, IPI
system interrupt can't be generated in mailbox driver. In such case
TF-A generates SGI to mailbox driver for IPI notification.

Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Signed-off-by: Saeed Nowshadi <saeed.nowshadi@amd.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>