PCI: rockchip-ep: Mark RK3399 as intx_capable

RK3399 can raise INTx interrupts, as can be seen by
rockchip_pcie_ep_send_intx_irq().

This is also in line with the register description of
PCIE_CLIENT_LEGACY_INT_CTRL, section "17.6.3 PCIe Client Detail Register
Description" of the RK3399 TRM.

Thus, mark RK3399 as intx_capable.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://patch.msgid.link/20250416142749.336298-2-cassel@kernel.org