[sw] Add X Macro Guidance
[sw] Add X Macro Guidance

This documents the way in which we use X Macros within the software tree
of OpenTitan - and how we will continue to use X Macros going forwards.

X Macros were originally added for the PMP library, and that addition
raised questions that this change seeks to answer.

Signed-off-by: Sam Elliott <32a4841301e2f444934d6e427a2764482fa7a43d@lowrisc.org>

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