PCI: designware-ep: Add the doorbell mode of MSI-X in EP mode

Add the doorbell mode of MSI-X in DWC EP driver.

Link: https://lore.kernel.org/r/20200918080024.13639-3-Zhiqiang.Hou@nxp.com


Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>