ARM: mx6: move to a standard arch/board approach

Freescale boards are currently all defined in arch/arm/Kconfig, which
makes them hard to detect.
Moreover the MX6 SoC variant (Q, D, DL, S, SL) selection is currently
done via the SYS_EXTRA_OPTIONS option which marked as deprecated.

Move to a more standard way to select sub-architecture and board by
creating a Kconfig under arch/arm/cpu/armv7/mx6 and a new ARCH_MX6
option.

Existing MX6 board definitions should be moved in this new Kconfig in
choice menu, and new boards should be directly declared in this menu.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>