PCI: dw-rockchip: Describe RK3588 BAR4 DMA ctrl window

On RK3588 PCIe3_4L in EP mode, the integrated DMA controller registers
are permanently mapped to BAR4 and must not be repurposed by EPF
drivers.

When the remote peer needs to access these registers, it must use the
fixed BAR4 window instead of creating another inbound mapping in a
different BAR. Mixing the fixed window with an additional mapping can
lead to incorrect behavior.

Advertise the DMA controller MMIO window as a reserved BAR subregion so
EPF drivers can reuse it safely.

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: Koichiro Den <den@valinux.co.jp>
Reviewed-by: Manikanta Maddireddy <mmaddireddy@nvidia.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260312130229.2282001-16-cassel@kernel.org

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