soc: fsl: guts: Add a NULL check for devm_kasprintf()

devm_kasprintf() may fail, so we should better add a NULL check
and propagate an error on failure.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>