ARM: shmobile: r8a7740: add FSI parent clock support

r8a7740 FSI can select its parent clock,
and its selection is dependent on platform board.

In order to enable FSI parent selection from platform board,
this patch adds its interface to clock framework.

This patch is based on v1.0 manual

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Simon Horman <horms@verge.net.au>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>