dts: arm: nxp: imx: Use 'nxp,imx-{i,d}tcm' compatible

On i.mx6/7 the TCMU and TCML regions are specific to instruction or
data.  So use the nxp,imx-itcm for TCML and nxp,imx-dtcm for TCMU.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>