ARM: shmobile: r8a7778: add SSI/SRU clock support

Add a platform clock for the r8a7778 SRU/SSI sound.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>