rtc: rv3028: add clkout support

rv3028 provides clkout (enabled by default). Add clkout
to clock framework source and control from device tree for
variable frequency with enable and disable functionality.

Signed-off-by: Parthiban Nallathambi <pn@denx.de>
Link: https://lore.kernel.org/r/20191018100425.1687979-1-pn@denx.de
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>