ARM: i.MX: remove unused ARCH_* configs

This removes the unused Kconfig options ARCH_MX5, ARCH_MX51,
ARCH_MX53 and MACH_MX21.

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