base: soc: Add serial_number attribute to soc

Add new attribute named "serial_number" as a standard interface for
user space to acquire the serial number of the device.

For ST-Ericsson SoCs this is exposed by the cryptically named "soc_id"
attribute, but this provides a human readable standardized name for this
property.

Tested-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Vaishali Thakkar <vaishali.thakkar@linaro.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Documentation/ABI/testing/sysfs-devices-soc

@@ -26,6 +26,13 @@ Description:
 		Read-only attribute common to all SoCs. Contains SoC family name
 		(e.g. DB8500).
 
+What:		/sys/devices/socX/serial_number
+Date:		January 2019
+contact:	Bjorn Andersson <bjorn.andersson@linaro.org>
+Description:
+		Read-only attribute supported by most SoCs. Contains the SoC's
+		serial number, if available.
+
 What:		/sys/devices/socX/soc_id
 Date:		January 2012
 contact:	Lee Jones <lee.jones@linaro.org>