Fix RNG interrupt priority

[jefftenney]

Fix RNG interrupt priority

Description

Prior to this PR, the RNG module's interrupt priority was 1, which is a higher priority than configMAX_SYSCALL_INTERRUPT_PRIORITY. After this PR, the RNG module's interrupt priority is 15, which is the lowest priority. Since the RNG interrupt handler makes FreeRTOS API calls, it must use a priority lower than configMAX_SYSCALL_INTERRUPT_PRIORITY.

Found this issue while using the latest CM33 port code (10.6.0) with this project. The latest port code includes validation of interrupt priorities.

Also:

• Updated FreeRTOSConfig.h to reflect that there are 4 priority bits implemented for interrupt priority.
• Changed the definition of configLIBRARY_LOWEST_INTERRUPT_PRIORITY from 7 to 15 (Note that configLIBRARY_LOWEST_INTERRUPT_PRIORITY is not actually used in this repo, so no impact from this change.)

Test Steps

Adopt kernel 10.6.0 into this repo. Prior to this PR, the application asserts due to the interrupt priority. After this PR, no assertion.

Checklist:

[X] I have tested my changes. No regression in existing tests. ~[X] I have modified and/or added unit-tests to cover the code changes in this Pull Request.~ N/A

Related Issue

(none)

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[jefftenney]

As a side note, I was using this repo to test port-optimised task selection on CM33 (https://github.com/FreeRTOS/FreeRTOS-Kernel/pull/703). It does work with only the minor change of setting configMAX_PRIORITIES to 31 (or 32) instead of 56. Just an FYI.