drivers: serial: nrfx_uarte: Optimize static peripheral configuration

When runtime configuration is not enable then we can determine at
compile time what are the hardware settings. Function which
translates zephyr values to nRF register values is not needed as
macros can do that at compile time.

This optimization saves almost 400 bytes of code.

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