ARM: shmobile: sh73a0 dtsi: Add SoC-specific FSI2 compatible property

The FSI2 sound node used the generic compatible property only.
Add the SoC-specific one, to make it future proof.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>