riscv: qemu: Enable PCI host ECAM generic driver

QEMU 4.0.0 'virt' target integrates a generic ECAM PCI host.
Enable the driver for it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Tested-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>