RISC-V: defconfig: Enable Generic PCIE by default

Enable generic PCIe by default in the RISC-V defconfig, this allows us
to use QEMU's PCIe support out of the box.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>