PCI: microchip: Add support for using either Root Port 1 or 2

The PCI host controller on PolarFire SoC has multiple Root Port instances,
each with their own bridge and ctrl address spaces. The original binding
has an "apb" register region, and it is expected to be set to the base
address of the Root Complex register space. Some defines in the Linux
driver were used to compute the addresses of the bridge and ctrl address
ranges corresponding to Root Port instance 1.  Some customers want to use
Root Port instance 2 however, which requires changing the defines in the
driver, which is clearly not a portable solution.

The binding has been changed from a single register region to a pair,
corresponding to the bridge and ctrl regions respectively, so modify the
driver to read these regions directly from the devicetree rather than
compute them from the base address of the abp region.

To maintain backwards compatibility with the existing binding, the driver
retains code to handle the "abp" reg and computes the base address of the
bridge and ctrl regions using the defines if it is present. reg-names has
always been a required property, so this is safe to do.

Link: https://lore.kernel.org/r/20241107-surrender-brisket-287d563a5de1@spud


Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
[bhelgaas: Capitalize PCIe spec terms]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>