watchdog: apseed: Add access_cs0 option for alt-boot

The option for the ast2400/2500 to get access to CS0 at runtime.
Signed-off-by: Ivan Mikhaylov <i.mikhaylov@yadro.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20190828102402.13155-5-i.mikhaylov@yadro.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

Documentation/ABI/testing/sysfs-class-watchdog

@@ -72,3 +72,37 @@ Description:
 		It is a read/write file. When read, the currently assigned
 		pretimeout governor is returned.  When written, it sets
 		the pretimeout governor.
+
+What:		/sys/class/watchdog/watchdog1/access_cs0
+Date:		August 2019
+Contact:	Ivan Mikhaylov <i.mikhaylov@yadro.com>,
+		Alexander Amelkin <a.amelkin@yadro.com>
+Description:
+		It is a read/write file. This attribute exists only if the
+		system has booted from the alternate flash chip due to
+		expiration of a watchdog timer of AST2400/AST2500 when
+		alternate boot function was enabled with 'aspeed,alt-boot'
+		devicetree option for that watchdog or with an appropriate
+		h/w strapping (for WDT2 only).
+
+		At alternate flash the 'access_cs0' sysfs node provides:
+			ast2400: a way to get access to the primary SPI flash
+				chip at CS0 after booting from the alternate
+				chip at CS1.
+			ast2500: a way to restore the normal address mapping
+				from (CS0->CS1, CS1->CS0) to (CS0->CS0,
+				CS1->CS1).
+
+		Clearing the boot code selection and timeout counter also
+		resets to the initial state the chip select line mapping. When
+		the SoC is in normal mapping state (i.e. booted from CS0),
+		clearing those bits does nothing for both versions of the SoC.
+		For alternate boot mode (booted from CS1 due to wdt2
+		expiration) the behavior differs as described above.
+
+		This option can be used with wdt2 (watchdog1) only.
+
+		When read, the current status of the boot code selection is
+		shown. When written with any non-zero value, it clears
+		the boot code selection and the timeout counter, which results
+		in chipselect reset for AST2400/AST2500.