arm/imx: remove imx_idle hook and use pm_idle instead

The patch removes imx_idle hook and use pm_idle instead to get imx
arch_idle prepared for the cleanup.  It's suggested by Russel King
as below.

> The final removal of mach/system.h depends on getting rid of the arch_idle
> thing.  While going through these headers, I was dismayed to find these:
>
> arch/arm/mach-s3c2410/include/mach/system.h:void (*s3c24xx_idle)(void);
> arch/arm/plat-mxc/include/mach/system.h:extern void (*imx_idle)(void);
>
> when we have a perfectly good pm_idle hook already in place - so there's
> no excuse for these especially when other platforms are already using
> pm_idle to hook their platform specific idle function into.  This is
> something that better be gone at the next merge window!

Suggested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>