PCI: microchip: Gather MSI information from hardware config registers

PCI: microchip: Gather MSI information from hardware config registers

The PCIe Root Complex on PolarFire SoC is configured at bitstream creation
time using Libero. Key MSI-related parameters include the number of
MSIs (1/2/4/8/16/32) and the MSI address. In the device driver, extract
this information from hardware registers at init time, and use it to configure
MSI system, including configuring MSI capability structure correctly in
configuration space.

Link: https://lore.kernel.org/r/20230728131401.1615724-7-daire.mcnamara@microchip.com
Signed-off-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>