drivers: i2c_nrfx_twim: add exclusive access API

This commit provides an extension to the i2c_nrfx_twim driver.
It introduces possibility to acquire/release exclusive access to the
i2c bus. While the exclusive access to the i2c bus is acquired
you can access the underlying nrfx_twim driver directly.

Signed-off-by: Andrzej Kuros <andrzej.kuros@nordicsemi.no>