soc: imx-scu: Using existing serial_number instead of UID

The soc_device_attribute structure already contains a serial_number
attribute to show SoC's unique ID, just use it to show SoC's unique
ID instead of creating a new file called soc_uid.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>