imx: cpu: move speed/temp to common cpu

The i.MX7 cpu speed/temp code could be reused on i.MX8M,
so move them to common cpu code.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>