MLK-18142 imx: timer: Modify GPT timer driver for mx7

MLK-18142 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@freescale.com>
(cherry picked from commit ff8902ab14705935a30f318db8d1c71ce3cc40dc)
(cherry picked from commit c3fa80efdb8680b4b35bc5b3607c148c3c48732b)
(cherry picked from commit 62625f0c8a2e90259488166ee8d8ddcf0ca4e9ae)