mx6slevk: Fix the pmic_get() parameter in the DM case

When pmic_get() is used with DM the first parameter must be
the complete node name plus the unit address, so fix it
accordingly.

Reported-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>