ARM: shmobile: emev2: Add clock tree description in DT

Add minimum clock tree description to .dts file.
This provides same set of clocks as current sh-clkfwk version .c
code does.

Signed-off-by: Takashi Yoshii <takasi-y@ops.dti.ne.jp>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>