[mask_rom] Add Mask ROM pseudocode.
[mask_rom] Add Mask ROM pseudocode.

Add the pseudo-code as a guide for further implementation.  The final
functional code is not necessarily expected to look like the pseudocode;
the pseudocode serves as a guide for the structure and implementation
of the actual mask ROM code.

Signed-off-by: Chris Frantz <d3397dc265da044816f611610217f391e4e7ef39@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan