x86: efi-x86_payload: Enable usb keyboard during boot

For boards that don't route serial port pins out, it's quite common
to attach a USB keyboard as the input device, along with a monitor.
However USB is not automatically started in the generic efi payload
codes. This uses a payload specific last_stage_init() to start the
USB bus, so that a USB keyboard can be used on the U-Boot shell.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>