ARM: imx: clean up machine mxc_arch_reset_init_dt reset init

System restart mechanism has been changed with the introduction
of "kernel restart handler call chain support". The imx2 watchdog
based restart handler has been moved to the driver, and these
restart can be removed from the machine layer.

This patch cleans up the device tree version machine reset init with
mxc_arch_reset_init_dt and removes corresponding .restart handler,
for the .init_machine that can be handled by system default after
removing the mxc_arch_reset_init_dt, the .init_machine is also removed.

Signed-off-by: Jingchang Lu <jingchang.lu@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>