drivers: serial: nrfx_uarte: Rename FAST to FAST_PD

Rename UARTE_ANY_FAST to UARTE_ANY_FAST_PD. There are 2 types of
'fast' UARTE instances. In nrf54h20 instance uart120 is in fast
power domain that requires additional power and clock management
of that domain. In nrf54lx fast uart00 instance does not require
that. Add _PD to indicate fast power domain.

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