ARM: i.MX: change timer clock from ipg to perclk

Contrary to the ipg clock the perclk rate is not changed or
gated in low power mode, so we choose perclk for gpt.

With the port to the common clock framework as a side effect
the timer used the rate returned from the peripheral clock
but the hardware was still programmed to use the ipg clock,
so this patch only changes the hardware to really use the
clock it already assumed.

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>