MAINTAINERS: group dts into respective platforms

currently all the dts changes of Texas Instruments platform
tags Simplelink due to the directory inclusion.

Split the dts inclusion for respective groups i.e K3, simplelink
and mspm0 is already correctly grouped.

MSP432 is currently not under any tree of maintainence.

Signed-off-by: Parthiban Nallathambi <parthiban@linumiz.com>