clk: shmobile: rcar-gen2: Use kick bit to allow Z clock frequency change

The Z clock frequency change is effective only after setting the kick
bit located in the FRQCRB register.
Without that, the CA15 CPUs clock rate will never change.

Fix that by checking if the kick bit is cleared and enable it to make
the clock rate change effective. The bit is cleared automatically upon
completion.

Signed-off-by: Benoit Cousson <bcousson+renesas@baylibre.com>
Acked-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>