clk: bd718x7: Initial support for ROHM bd71837/bd71847 PMIC clock

ROHM bd71837 and bd71847 contain 32768Hz clock gate. Support the clock
using generic clock framework. Note, only bd71837 is tested but bd71847
should be identical what comes to clk parts.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>