driver core: Add state_synced sysfs file for devices that support it

This can be used to check if a device supports sync_state() callbacks
and therefore keeps resources left on by the bootloader enabled till all
its consumers have probed.

This can also be used to check if sync_state() has been called for a
device or whether it is still trying to keep resources enabled because
they were left enabled by the bootloader and all its consumers haven't
probed yet.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20200521191800.136035-3-saravanak@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-devices-state_synced

@@ -0,0 +1,24 @@
+What:		/sys/devices/.../state_synced
+Date:		May 2020
+Contact:	Saravana Kannan <saravanak@google.com>
+Description:
+		The /sys/devices/.../state_synced attribute is only present for
+		devices whose bus types or driver provides the .sync_state()
+		callback. The number read from it (0 or 1) reflects the value
+		of the device's 'state_synced' field. A value of 0 means the
+		.sync_state() callback hasn't been called yet. A value of 1
+		means the .sync_state() callback has been called.
+
+		Generally, if a device has sync_state() support and has some of
+		the resources it provides enabled at the time the kernel starts
+		(Eg: enabled by hardware reset or bootloader or anything that
+		run before the kernel starts), then it'll keep those resources
+		enabled and in a state that's compatible with the state they
+		were in at the start of the kernel. The device will stop doing
+		this only when the sync_state() callback has been called --
+		which happens only when all its consumer devices are registered
+		and have probed successfully. Resources that were left disabled
+		at the time the kernel starts are not affected or limited in
+		any way by sync_state() callbacks.
+
+