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>

include/dm/uclass-id.h

@@ -90,6 +90,7 @@ enum uclass_id {
 	UCLASS_POWER_DOMAIN,	/* (SoC) Power domains */
 	UCLASS_PWM,		/* Pulse-width modulator */
 	UCLASS_PWRSEQ,		/* Power sequence device */
+	UCLASS_QFW,		/* QEMU firmware config device */
 	UCLASS_RAM,		/* RAM controller */
 	UCLASS_REGULATOR,	/* Regulator device */
 	UCLASS_REMOTEPROC,	/* Remote Processor device */