PCI: endpoint: Describe reserved subregions within BARs

PCI: endpoint: Describe reserved subregions within BARs

Some endpoint controllers expose platform-owned, fixed register windows
within a BAR that EPF drivers must not reprogram (e.g. a BAR marked
BAR_RESERVED). Even in that case, EPF drivers may need to reference a
well-defined subset of that BAR, e.g. to reuse an integrated DMA
controller MMIO window as a doorbell target.

Introduce struct pci_epc_bar_rsvd_region and extend struct
pci_epc_bar_desc so EPC drivers can advertise such fixed subregions in a
controller-agnostic way.

No functional change for existing users.

Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Tested-by: Manikanta Maddireddy <mmaddireddy@nvidia.com>
Tested-by: Koichiro Den <den@valinux.co.jp>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260312130229.2282001-15-cassel@kernel.org