drivers/pcie: Improve and fix MBAR retrieval depending on use cases

So far pcie_get_mbar() has been the only way to retrieve a MBAR. But
it's logic does not fit all uses cases as we will see further.
The meaning of its parameter "index" is not about BAR index but about
a valid Base Address count instead. It's an arbitrary way to index
MBARs unrelated to the actual BAR index.

While this has proven to be just the function we needed so far, this has
not been the case for MSI-X, which one (through BIR info) needs to
access the BAR by their actual index. Same as ivshmem in fact, though
that one did not generate any bug since it never has IO BARs nor 64bits
BARs (so far?).

So:

- renaming existing pcie_get_mbar() to pcie_probe_mbar(), which is a
  more relevant name as it indeed probes the BARs to find the nth valid
  one.
- Introducing a new pcie_get_mbar() which this time really asks for the
  BAR index.
- Applying the change where relevant. So all use pcie_probe_mbar() now
  but MSI-X and ivshmem.

Fixes #37444

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>

drivers/serial/uart_ns16550.c

@@ -360,7 +360,7 @@ static int uart_ns16550_configure(const struct device *dev,
 			goto out;
 		}
 
-		pcie_get_mbar(dev_cfg->pcie_bdf, 0, &mbar);
+		pcie_probe_mbar(dev_cfg->pcie_bdf, 0, &mbar);
 		pcie_set_cmd(dev_cfg->pcie_bdf, PCIE_CONF_CMDSTAT_MEM, true);
 
 		device_map(DEVICE_MMIO_RAM_PTR(dev), mbar.phys_addr, mbar.size,