drivers: serial: nrfx_uarte: Request hsfll clock for fast instance

Request fast global domain to run at 320 MHz during fast UARTE
activity. As request is asynchronous it cannot be called from
an ISR. Due to complexity to handle that without device runtime
power management a requirement is added so that if fast UARTE
is used device runtime PM must be enabled. Clock is request
and released in PM resume and suspended actions which in case
of fast UARTE are only called from thread context.

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