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: NVinod Koul <vkoul@kernel.org>
Signed-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: NVaishali Thakkar <vaishali.thakkar@linaro.org>
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: NStephen Boyd <swboyd@chromium.org>
Reviewed-by: NVinod Koul <vkoul@kernel.org>
Signed-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>