PCI: dwc: rcar-gen4: Change EPC BAR alignment to 4K as per the documentation

PCI: dwc: rcar-gen4: Change EPC BAR alignment to 4K as per the documentation

R-Car S4 Series (R8A779F[4-7]*) EP controller uses a 4K minimum iATU region
size (CX_ATU_MIN_REGION_SIZE = 4K) as per R19UH0161EJ0130 Rev.1.30. Also,
the controller itself can only be configured in the range 4 KB to 64 KB, so
the current 1 MB alignment requirement is incorrect.

Hence, change the alignment to the min size 4K as per the documentation.

This also fixes needless unusability of BAR4 on this platform when the
target address is fixed, such as for doorbell targets.

Fixes: e311b3834dfa ("PCI: rcar-gen4: Add endpoint mode support")
Signed-off-by: Koichiro Den <den@valinux.co.jp>
[mani: commit log]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20260305151050.1834007-1-den@valinux.co.jp