zynq7000: initialize all CPUs

[jmaksymowicz]

This change allows all CPU cores to jump into kernel on zynq7000 (both real hardware and QEMU).

Description

The procedure of starting other CPU cores on zynq7000 is substantially different on QEMU and real hardware. On QEMU both cores jump to start of PLO code. On real hardware boot ROM parks CPU 1 in a small bit of code in high OCRAM that monitors memory address 0xfffffff0 and sleeps using wfe. In order to achieve the same behavior on QEMU code was added into _init.S that parks all other CPU cores in the same fashion.

After change CPU 0 signals other cores that they should jump to kernel before jumping to kernel itself.

Motivation and Context

Required to allow use of SMP kernel on zynq7000.

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-zturn, 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   39m 39s ⏱️ +26s
  420 suites ±0     715 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit ccfb38b. ± Comparison against base commit f923d7c.

♻️ This comment has been updated with latest results.

[agkaminski]

LGTM