rtc: pcf8563: Constify clkout_rates

The lates of clockout should be marked const. Make that so.

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Link: https://lore.kernel.org/r/20191108002449.15097-1-iwamatsu@nigauri.org


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>