x86: qemu: move QFW to its own uclass

We move qfw into its own uclass and split the PIO functions into a
specific driver for that uclass.  The PIO driver is selected in the
qemu-x86 board config (this covers x86 and x86_64).

include/qfw.h is cleaned up and documentation added.

Signed-off-by: Asherah Connor <ashe@kivikakk.ee>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/board/emulation/qemu-x86/Kconfig b/board/emulation/qemu-x86/Kconfig
index 6d19299..9bb8a72 100644
--- a/board/emulation/qemu-x86/Kconfig
+++ b/board/emulation/qemu-x86/Kconfig
@@ -20,6 +20,7 @@
 	def_bool y
 	select X86_RESET_VECTOR
 	select QEMU
+	select QFW_PIO
 	select BOARD_ROMSIZE_KB_1024
 	imply VIRTIO_PCI
 	imply VIRTIO_NET