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>

include/linux/sys_soc.h

@@ -12,6 +12,7 @@ struct soc_device_attribute {
 	const char *machine;
 	const char *family;
 	const char *revision;
+	const char *serial_number;
 	const char *soc_id;
 	const void *data;
 };