riscv: dts: microchip: mpfs: remove ti,fifo-depth property

Recent versions of dt-schema warn about a previously undetected
undocument property on the icicle & polarberry devicetrees:

arch/riscv/boot/dts/microchip/mpfs-icicle-kit.dtb: ethernet@20112000: ethernet-phy@8: Unevaluated properties are not allowed ('ti,fifo-depth' was unexpected)
        From schema: Documentation/devicetree/bindings/net/cdns,macb.yaml

I know what you're thinking, the binding doesn't look to be the problem
and I agree. I am not sure why a TI vendor property was ever actually
added since it has no meaning... just get rid of it.

Fixes: bc47b221 ("riscv: dts: microchip: add the sundance polarberry")
Fixes: 0fa6107e

 ("RISC-V: Initial DTS for Microchip ICICLE board")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>