PCI: dwc: Use resource start as ioremap() input in dw_pcie_pme_turn_off()

The msg_res region translates writes into PCIe Message TLPs. Previously we
mapped this region using atu.cpu_addr, the input address programmed into
the ATU.

"cpu_addr" is a misnomer because when a bus fabric translates addresses
between the CPU and the ATU, the ATU input address is different from the
CPU address.  A future patch will rename "cpu_addr" and correct the value
to be the ATU input address instead of the CPU physical address.

Map the msg_res region before writing to it using the msg_res resource
start, a CPU physical address.

Link: https://lore.kernel.org/r/20250315201548.858189-2-helgaas@kernel.org


Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>