riscv: dts: microchip: make the fabric dtsi board specific

riscv: dts: microchip: make the fabric dtsi board specific

Currently mpfs-fabric.dtsi is included by mpfs.dtsi - which is fine
currently since there is only one board with this SoC upstream.

However if another board was added, it would include the fabric contents
of the Icicle Kit's reference design. To avoid this, rename
mpfs-fabric.dtsi to mpfs-icicle-kit-fabric.dtsi & include it in the dts
rather than mpfs.dtsi.

mpfs-icicle-kit-fabric.dtsi specifically matches the 22.03 reference
design for the icicle kit's FPGA fabric & an older version of the
design may not have the i2c or pwm devices - so add the compatible
string to document this.

Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220509142610.128590-6-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>