ARM i.MX25 clk: Fix gpt timer clock.

The gpt0 timer clock has been wrong since:
  6bbaec56

 ARM i.MX25: implement clocks using common clock framework

Signed-off-by: Denis Carikli <denis@eukrea.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>