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/common/Makefile b/common/Makefile
index 215b8b2..0952ae2 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -138,3 +138,5 @@
 
 obj-$(CONFIG_AVB_VERIFY) += avb_verify.o
 obj-$(CONFIG_SCP03) += scp03.o
+
+obj-$(CONFIG_QFW) += qfw.o