mxc: add arch_reset() function

This patch adds arch_reset() function for all mxc platforms.

It also removes (unsused) arch/arm/mach-mx2/system.c file.

This patch has been tested on i.MX1/27/31/35

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>