clk: bd718x7: Add support for clk gate on ROHM BD71815 PMIC

ROHM BD71815 also provide clk signal for RTC. Add control
for gating this clock.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>