Use memcpy in nr_prach.c to fill txdata buffer.

Remove global tables related to PRACH generation.

Add new function to compute modulo multiplicative inverse and use it
directly to get inverse instead of LUT.

Removed nr_ru table and compute the values in-place.

In UE, added new flag root_seq_computed in nrUE_config to only compute
the sequence once after new nrUE_config is received.