Define ARCH_MXC for i.MX devices. This is useful to identify features or
behaviors common to all i.MX SoCs.

The i.MX28 is omitted because its architecture is a bit different (like imx/mxc
vs. mxs in Linux).

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Andy Fleming <afleming@gmail.com>
Cc: Kim Phillips <kim.phillips@freescale.com>