PCI: rcar-gen4: set ep BAR4 fixed size

On rcar-gen4, the ep BAR4 has a fixed size of 256B. Document this
constraint in the epc features of the platform.

Fixes: e311b383 ("PCI: rcar-gen4: Add endpoint mode support")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://patch.msgid.link/20250328-rcar-gen4-bar4-v1-1-10bb6ce9ee7f@baylibre.com