LFSR update priorities wrong

Created by: Silabs-ArjanB

The LFSR update priorities are wrong, they should be as in https://github.com/lowRISC/ibex/blob/410ffd349da1d4c13f77b208bb3b93adb154fac0/vendor/lowrisc_ip/ip/prim/rtl/prim_lfsr.sv#L380

In the current code LFSR writes by software would be ignored unless they happen to coincide with a hardware based LFSR write.

Priorities should be:

1. seed_we_i
2. enable_i && lockup_o
3. enable_i

RTL should also be modified such that lfsr_q will get clock gated unless (seed_we_i || enable_i).