ARM: shmobile: r8a7778: remove legacy clock implementation

Bock-W was the last legacy clock platform, so this also removes the
common legacy clock code.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Acked-by: by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>