[rram_ctrl,sw] constant runtime for crypto functions

The following tests expect a constant runtime for sub functions (e.g. hardened_xor). This cannot be guaranteed if we fetch instructions from the RRAM.
```
//sw/device/lib/base:hardened_memory_functest_fpga_cw340_sival_rom_ext
//sw/device/tests/crypto:aes_gcm_timing_functest_fpga_cw340_sival_rom_ext
//sw/device/lib/base:hardened_memory_functest_fpga_cw340_rom_with_fake_keys
//sw/device/tests/crypto:aes_gcm_timing_functest_fpga_cw340_rom_with_fake_keys
```

The read-time is not constant due to:
- double read from RRAM with verification read that can stall instruction fetch if the read-buffer is full
- not guaranteed read latency

Start with the four affected test targets listed in the issue, especially the hardened memory and AES-GCM timing tests. Read how they measure or assert constant runtime, then compare those expectations with the documented RRAM double-read and variable-latency behavior. Done means the affected tests account for the stated hardware timing constraints and pass.

Written by the indexing model from the issue text.