dt_bindings: bd71828: Add clock output mode

The BD71828 allows configuring the clk32kout pin mode to CMOS or
open-drain. Add device-tree property for specifying the preferred mode.

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