armv7a9-zynq7000-qemu: disable SMP

[jmaksymowicz]

Due to poor performance under QEMU when SMP kernel is enabled, change device tree used by QEMU to a single-core system.

Description

Without this change QEMU would run very slowly after the SMP kernel is implemented. Single-thread workloads such as reading from emulated Flash would slow down by about 5x in internal testing. This slowdown is caused by an interaction between idle thread implementation in our kernel and inefficiencies in QEMU CPU and timer emulation. This slowdown only occurs on QEMU, not on real hardware. Because our CI testing setup already includes physical zynq7000 devices, I think limiting QEMU to only emulate one core is a reasonable solution.

Motivation and Context

Without this change some CI tests on armv7a9-zynq7000-qemu would trigger timeouts. It is needed to merge phoenix-rtos/phoenix-rtos-kernel#548

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: armv7a9-zynq7000-qemu

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[github-actions]

Unit Test Results

7 460 tests  ±0   6 745 ✅ ±0   37m 53s ⏱️ - 2m 36s
  420 suites ±0     715 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 623f9e1. ± Comparison against base commit 5eabbc3.