RTEMS | Add aarch64 cpukit paravirtualization support (!211)

[Joel Sherrill (@joel)]

Joel Sherrill commented on a discussion on cpukit/score/cpu/aarch64/include/rtems/score/paravirt.h: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/211#note_112133

 > +#endif
 > +
 > +#ifndef _RTEMS_SCORE_PARAVIRT_H
 > +#define _RTEMS_SCORE_PARAVIRT_H
 > +
 > +/**
 > + * @defgroup RTEMSScoreCPUARMParavirt ARM Paravirtualization Support
 > + *
 > + * @ingroup RTEMSScoreCPUARM
 > + *
 > + * This handler encapulates the functionality (primarily conditional
 > + * feature defines) related to paravirtualization on the ARM.
 > + *
 > + * Paravirtualization on the ARM makes the following assumptions:
 > + *
 > + *   - RTEMS executes in user space

This came from Deos support:

> User mode code runs in EL0. Kernel mode code runs in EL1. The boot, PAL (kernel mode driver) and PRLs (Platform resource libraries) are considered kernel mode code.
We do have some requirements for boot to do some initialization of EL2/EL3 registers in the desk/help/deos-boot-software-requirements.htm, but I believe those will have to be revisited for running under a hypervisor as most of them prevent returning to EL2/EL3 and allow EL1 full access to secure registers (when possible) and the entire GIC.

Hopefully that clarifies things for you and @opticron

-- 
View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/211#note_112133
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