drivers: serial: nrfx_uarte: Add support for higher baudrates

nrf54x devices have UARTE instance capable of using baudrate higher
than 1M. Higher baudrates does not have predefined values for
BAUDRATE register. A formula can be used to calculate BAUDRATE
value that shall be used for desired baudrate. Add UARTE_ANY_HIGH_SPEED
macro which is set when high speed is enabled (uart00 in nrf54lx or
uart120 in nrf54h20). For high speed instance use formula for getting
value that shall be written to BAUDRATE register.

When runtime configuration is not used then same formula is used to
calculate fixed BAUDRATE value.

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