riscv: qemu: Enable usb keyboard as an input device

This brings PCI xHCI support to QEMU RISC-V and uses a usb keyboard
as one of the input devices.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Simon Glass <sjg@chromium.org>