ARM: shmobile: r8a7791 dtsi: Add CPG/MSTP Clock Domain for sound

797a0626 ("ARM: shmobile: r8a7791 dtsi: Add CPG/MSTP Clock Domain")
added CPG/MSTP clock-cells domain support, but it was missing sound
support. This patch adds it.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
[horms: updated commit id referred to in changelog]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>