arm/imx: remove cpu_is_xxx() from arch_idle()

This patch adds an idle hook imx_idle to be called in arch_idle().
Any soc that needs a customized idle implementation other than
cpu_do_idle() can set up this hook in soc specific call.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>