PCI: Move pci_rebar_bytes_to_size() and clean it up

PCI: Move pci_rebar_bytes_to_size() and clean it up

Move pci_rebar_bytes_to_size() from include/linux/pci.h to rebar.c as it
does not look very trivial and is not expected to be performance critical.

Convert literals to use a newly added PCI_REBAR_MIN_SIZE define.

Also add kernel doc for the function as the function is exported.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michael J. Ruhl <mjruhl@habana.ai>
Link: https://patch.msgid.link/20251113180053.27944-3-ilpo.jarvinen@linux.intel.com