drivers: serial: nrfx_uarte: Add workaround for bytes dropping

Add workaround for development kits on which interface chip (segger chip)
is dropping bytes when they are sent in bursts. Issue has been seen on
DK which have multiple virtual com ports on the USB.

A workaround is adding periodic busy waits to introduce gaps in the
transmission.

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