ARM: dts: i.MX25: Fix gpt timers clocks.

Without that fix, the board freeze during boot.

This appeared after the following commit:
  496f065 ARM: i.MX: Use of_clk_get_by_name() for timer clocks for DT case.

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