ARM: mxs: convert stmp378x_devb board to device tree

The enc28j60 spi device does not get converted to dts file, because
there is no mxs spi driver on mainline so far.  The enc28j60 spi
in mach-stmp378x_devb.c does not work anyway.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>