ARM: shmobile: r8a73a4: safeguard against wrong clk_set_rate() uses

clk_set_rate() should only be called with exact rates, returned by
clk_round_rate(). However, it is still good to verify, that the value,
passed to clock's .set_rate() method is at least valid. This patch adds
such a check for the Z-clock on r8a73a4.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>