ARM: imx: create mxc_arch_reset_init_dt() for DT boot

The mxc_arch_reset_init() uses static mapping and calls clk_get_sys() to
get clock.  It's suitable for non-DT boot but not for DT boot where
dynamic mapping and of_clk_get() should be used instead.  Create
mxc_arch_reset_init_dt() as the DT variant of mxc_arch_reset_init(),
and change DT platforms to use it.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>