ENGR00220154 GPT mx6: move mx6_timer_rate to clock.c

ENGR00220154 GPT mx6: move mx6_timer_rate to clock.c

System will report oops as below. To fix it we will move mx6_timer_rate to
clock.c, so that we can avoid use clk_get_sys which cause schedule after
spin_lock.

oops log:
BUG: scheduling while atomic: kinteractiveup/1403/0x00000002
Modules linked in:
(unwind_backtrace+0x0/0xfc) from [<804f05f0>] (__schedule+0x4b8/0x6b0)
(__schedule+0x4b8/0x6b0) from [<804f12ac>] (__mutex_lock_slowpath+0x138/0x208)
(__mutex_lock_slowpath+0x138/0x208) from [<804f13b4>] (mutex_lock+0x38/0x3c)
mutex_lock+0x38/0x3c) from [<803b9134>] (clk_get_sys+0x1c/0xec)
(clk_get_sys+0x1c/0xec) from [<8005f814>] (mx6_timer_rate+0x14/0x7c)
(mx6_timer_rate+0x14/0x7c) from [<80056a20>] (_clk_gpt_get_rate+0x18/0x2c)
(_clk_gpt_get_rate+0x18/0x2c) from [<8005e89c>] (clk_get_rate+0x34/0x40)
(clk_get_rate+0x34/0x40) from [<80055f3c>] (_clk_pll_enable+0xa8/0x1ec)
(_clk_pll_enable+0xa8/0x1ec) from [<80056088>] (_clk_pll1_enable+0x8/0x20)
(_clk_pll1_enable+0x8/0x20) from [<80056998>] (_clk_arm_set_rate+0x278/0x2e8)
(_clk_arm_set_rate+0x278/0x2e8) from [<8005e824>] (clk_set_rate+0x54/0x68)
(clk_set_rate+0x54/0x68) from [<80061660>] (set_cpu_freq+0xb8/0x160)
(set_cpu_freq+0xb8/0x160) from [<800618b4>] (mxc_set_target+0xf0/0x20c)
(mxc_set_target+0xf0/0x20c) from [<80372388>](__cpufreq_driver_target+0x54/0x60)
Signed-off-by: Robin Gong <b38343@freescale.com>