clk: imx31: Do not call mxc_timer_init twice when booting with DT

mxc_timer_init must not be called from within mx31_clocks_init_dt. It will
eventually be called by imx31_timer_init_dt
(drivers/clocksource/timer-imx-gpt.c).
This arranges the initialization code similar to clk-imx27.c

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>