pmic: Introduce power_init_board() method at ./lib/board.c file

It is necessary to introduce a new system wide function- power_init_board()

It turns out, that power initialization must be done as early as possible.
In the case of PMIC framework redesign, which aims to support multiple
instances of PMIC devices the initialization shall be performed just
after malloc configuration.

The power_init_board function is a weak function with default implementation.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>