MLK-18243-3 arm: imx8m: add clocks definition for i.MX8MM

Add clock definition for i.MX8MM
Move the original definition in clock.h to clock_imx8mq.h
Use CONFIG_IMX8MQ/M in clock.h to choose the correct header file.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>