dts/riscv: move `mpfs-icicle.dtsi` into a common `microchip` directory

This commit moves the `mpfs-icicle.dtsi` file to a common `microchip`
directory and updates include paths in the `mpfs_icicle` board
devicetree.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>