pci_endpoint_test: Add 2 ioctl commands

Add MSI-X support and update driver documentation accordingly.

Add 2 new IOCTL commands:
 - Allow to reconfigure driver IRQ type in runtime.
 - Allow to retrieve current driver IRQ type configured.

Add IRQ type validation before executing the READ/WRITE/COPY tests.

Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>