ARM: shmobile: r8a7778: tidyup DT node naming

According to ePAPR spec,
this patch tidies up DT node name and related clock.

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