PM / Hibernate: Add resumewait param to support MMC-like devices as resume file

Some devices like MMC are async detected very slow. For example,
drivers/mmc/host/sdhci.c launches a 200ms delayed work to detect
MMC partitions then add disk.

We have wait_for_device_probe() and scsi_complete_async_scans()
before calling swsusp_check(), but it is not enough to wait for MMC.

This patch adds resumewait kernel param just like rootwait so
that we have enough time to wait until MMC is ready. The difference is
that we wait for resume partition whereas rootwait waits for rootfs
partition (which may be on a different device).

This patch will make hibernation support many embedded products
without SCSI devices, but with devices like MMC.

[rjw: Modified the changelog slightly.]

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Reviewed-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

Documentation/kernel-parameters.txt

@@ -2240,6 +2240,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			in <PAGE_SIZE> units (needed only for swap files).
 			See  Documentation/power/swsusp-and-swap-files.txt
 
+	resumewait	[HIBERNATION] Wait (indefinitely) for resume device to show up.
+			Useful for devices that are detected asynchronously
+			(e.g. USB and MMC devices).
+
 	hibernate=	[HIBERNATION]
 		noresume	Don't check if there's a hibernation image
 				present during boot.