PCI: Add pci_rebar_bytes_to_size()

Users of pci_resize_resource() need a way to calculate BAR size
from desired bytes. Add a helper function and export it so that
modular drivers can use it.

Signed-off-by: Darren Salt <devspam@moreofthesa.me.uk>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20210107175017.15893-3-nirmoy.das@amd.com