Add Chipyard port of Zephyr RTOS

[CobbledSteel]

Summary:
Add initial support of Zephyr RTOS to Chipyard. Intended to be an intermediate option between Linux/FireMarshal flow and BaremetalIDE flow. Evaluated on Spike and RTL sim with RocketConfig/DualRocketConfig.

Zephyr RTOS Integration:

• Adds Zephyr as a submodule (chipyard/software/zephyr)
• Implements west-based build system for Spike and Chipyard RTL simulation
• Enables Zephyr boot (~20K cycles on RocketConfig) for lightweight RTOS support

HTIF UART Driver for Spike/FESVR:

• Implements Zephyr UART driver for HTIF-based serial I/O
• Supports console and shell via HTIF
• Integrated into Zephyr device tree and KConfig

Documentation Updates:

• Adds Zephyr Getting Started guide for Chipyard
• Documents Zephyr driver structure and integration
• Provides step-by-step guide for adding new Zephyr drivers

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

Contributor Checklist:

• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

CI Help:
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• ci:fpga-deploy - Run FPGA-based E2E testing
• ci:local-fpga-buildbitstream-deploy - Build local FPGA bitstreams for platforms that are released
• ci:disable - Disable CI

[jerryz123]

If this can boot in 20k instructions, we should add a CI test that checks it using RocketConfig.

[jerryz123]

Zephyr is a gigantic repo, it significantly degrades clone time of chipyard.

Can you modify scripts/init-submodules-no-riscv-tools-nolog.sh such that that script does not initialize Zephyr? You can follow the pattern of the existing repos on the "blocklist" for the recursive init.

Then indicate the user should manually git submodule update --init zephyr in the docs