ARM: shmobile: r8a7779: add HSPI clock support for DT

Based on work for the r8a7778 SoC by Kuninori Morimoto.

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