ARM: shmobile: sh73a0: do not overwrite all div4 clock operations

An earlier commit "ARM: shmobile: sh73a0: add support for adjusting CPU
frequency" intended to replace some clock operations only for the Z-clock,
instead it replaced them for all div4 clocks, since all div4 clocks share
the same copy of clock operations. Fix this by using a separate clock
operations structure for Z-clock.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>