[dv] Refactor mem_bkdr_util

As @sriyerg [suggested](https://github.com/lowRISC/opentitan/pull/16878#discussion_r1051110782), it's better to implement those helper files (mem_bkdr_util__rom/flash/otp/sram) as derived classes such as otp_bkdr_util, flash_bkdr_util etc.

Right now, they are all included in the same class `mem_bkdr_util`, but for example, otp doesn't need the scramble function of sram, likewise to other mem blocks.

It may take a while to refactor it since this class has been widely used in 4 blocks and chip-level as well as close source repo.
Besides making the class structure better, I don't see it has other immediate benefit. Hence, it sounds a P3/P4 issue.

Start by reading mem_bkdr_util and the helper files mem_bkdr_util__rom/flash/otp/sram, then trace their use across the four memory blocks and chip-level code. Refactor the helpers into derived classes such as otp_bkdr_util and flash_bkdr_util, update affected usages, and verify that each memory block retains only the relevant behavior.

Written by the indexing model from the issue text.