[nrf fromtree] drivers: serial: nrfx_uarte: Split async control block structure

Split async control block structure into tx and rx structures.

Signed-off-by: Krzysztof Chruściński <[email protected]>
(cherry picked from commit 701beec)
Signed-off-by: Rafał Kuźnia <[email protected]>

Author: nordic-krch