PCI: endpoint: Add pci_epc_bar_size_to_rebar_cap()

PCI: endpoint: Add pci_epc_bar_size_to_rebar_cap()

Add a helper function to convert a size to the representation used by the
Resizable BAR Capability Register.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20250131182949.465530-11-cassel@kernel.org
[mani: squashed the change that added PCIe spec reference to comments
from https://lore.kernel.org/linux-pci/20250219171454.2903059-2-cassel@kernel.org]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>