imx: timer: Modify GPT timer driver for mx7

Modify the GPT common platform driver for mx7 which only use 24Mhz
OSC as clock source.

Note: at default, the mx7d will use system counter as timer. The GPT
is disabled.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>