dts: arm: microchip: Organize MEC parts into a subfolder

Microchip is adding more non-MEC devices to Zephyr such as
SAM and PIC32. Each device family will have its own subfolder.
We moved the existing MEC DTSI files into a new mec subfolder.
We also updated the existing MEC boards include paths.

Signed-off-by: Scott Worley <scott.worley@microchip.com>