rtc: pcf8563: add CLKOUT to common clock framework

Add the clkout output clk to the common clock framework.
Disable the CLKOUT of the RTC after power-up.
After power-up/reset of the RTC, CLKOUT is enabled by default,
with CLKOUT enabled the RTC chip has 2-3 times higher power
consumption.

Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>