Add `isys_rom` module
This pull request adds the InfraSYS ROM module, analogous to the MSYS ROM module, for bootloading InfraSYS-based systems. This replaces the custom bootloading logic used by both SGI-575 and RD-N1-E1.
@manojkumar-arm, @omasab, @masahisak, @tusharkhandelwal2: we've added support for this module to SGI-575 and RD-N1-E1. This PR may be of interest to you if you are using a similar booting mechanism to these platforms, in which case you can use these platforms' changes as a reference.