PCI: dwc: Export DWC MSI controller related APIs

PCI: dwc: Export DWC MSI controller related APIs

Export dw_pcie_msi_host_init(), dw_pcie_msi_init(), and dw_pcie_free_msi()
APIs to allow them to be reused by the upcoming DWC based ECAM driver
implementation. Also, move MSI IRQ related initialization code to
dw_pcie_msi_init(), as this code must be executed before dw_pcie_msi_init()
API can be used with ECAM driver.

Signed-off-by: Mayank Rana <mayank.rana@oss.qualcomm.com>
[mani: commit message rewording]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250616224259.3549811-2-mayank.rana@oss.qualcomm.com