ARM: mxs: remove board files

Since DT kernel provides the equivalent support with those board
files, we can remove them now.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Lothar Waßmann <LW@KARO-electronics.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Lauri Hintsala <lauri.hintsala@bluegiga.com>