ARM: dts: mxs: Add descriptions for imx287 based btt3-[012] devices

ARM: dts: mxs: Add descriptions for imx287 based btt3-[012] devices

The btt3 device' HW revisions from 0 to 2 use imx287 SoC and are to
some extend similar to already upstreamed XEA devices, hence are
using common imx28-lwe.dtsi file.

New, imx28-btt3.dtsi has been added to embrace common DTS
properties for different HW revisions for this device.

As a result - changes introduced in imx28-btt3-[012].dts are
minimal.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>