soc: nordic: common: dmm: Initialize dmm as early as possible

DMM shall be initialized as early as possible to allow drivers to
use it. For example, uart may need it early since it starts
RX during initilization in some configurations.

Making dmm_init() public and calling it in soc init function.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>