imx: cpu: add cpu speed/grade for i.MX8M

Use more general enum types for speed grade.
Refine get_cpu_speed_grade_hz to support i.MX8M.

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>