PCI: microchip: Rename PLDA structures to be generic

Rename struct mc_msi to plda_msi and move most of struct mc_pcie to a new
struct plda_pcie_rp so they can be shared by all PLDA-based drivers.

The axi_base_addr field remains in struct mc_pcie since it's
Microchip-specific data.

The event interrupt code is still using struct mc_pcie because the event
interrupt code can not be re-used.

[kwilczynski: commit log]
Link: https://lore.kernel.org/linux-pci/20240328091835.14797-6-minda.chen@starfivetech.comSigned-off-by: Minda Chen <minda.chen@starfivetech.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>