ARM: shmobile: r7s72100: add essential clock nodes to dtsi

Only essential clocks are added for now. Other clocks will be added when
needed.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>