Microchip: MEC172X DTS files reorganization

MEC172X series SoCs share most IP but the -LJ series expands the PWM and
ADC channels available as well as defines extra pinctrl pins.
Separating these better to be able to simplify their inclusion and
driver code.  Any board based on either the -SZ or -LJ package can just
include the mec172x<sz/lj> dtsi files for their specific package.

Signed-off-by: Jeff Daly <jeffd@silicom-usa.com>