R2S Plus-support (with regressionstest)

[pjator]

Description

Add support for the NanoPi R2S Plus, which adds an on-board 32 GB eMMC
to the plain R2S. The SoC, DRAM and network layout are identical to the
R2S — the only difference is the eMMC, so this is a superset variant
rather than a separate board.

Changes:

• Device tree: add rk3328-nanopi-r2s-plus.dts, wrapping the
  kernel's rk3328-nanopi-r2s-plus (which builds on the shared
  rk3328-nanopi-r2s.dtsi), and list both .dtbs in dts/Makefile,
  following the bpi-r3 pattern. The wrapper overrides the top-level
  compatible to friendlyarm,nanopi-r2s, so the Plus shares the R2S
  product configuration — no separate product directory is needed.

• U-Boot DT selection: the R2S had no fdtfile/ixvariant
  mechanism, so U-Boot always loaded the plain r2s DT and the kernel
  never saw the eMMC. Add ixvariant (bpi-r4 pattern) that selects
  the plus DT when mmc0 (eMMC) is present, falling back to the r2s
  DT otherwise.

• ChangeLog: entry under v26.08.0.

Regression Testing (Infamy, physical hardware)

Ran against the NanoPi R2S Plus in test mode
(touch /mnt/aux/test-mode && reboot), using the official Infamy
test suite with a single-DUT topology (controller host + target):

• case/hardware/watchdog: 6/6 pass — watchdog tripped on an
  injected hard lockup (all cores, IRQs disabled) and rebooted the
  system back to a working state
• case/system (hostname, add/delete user, admin user, NACM,
  timezone x2, schedule reboot): 7/7 pass
• case/system/factory_config: skipped in test mode by design (test
  mode always boots the test config, never the factory config)
• case/system/upgrade: not run (requires a data port not wired in
  this lab setup)

Found and worked around an upstream bug in the test harness while
running these tests (Topology.get_password() qstrip NameError) —
fixed in a separate PR (#1588).

Checklist

Tick relevant boxes, this PR is-a or has-a:

• Bugfix
  • Regression tests
  • ChangeLog updates (for next release)
• Feature
  • YANG model change => revision updated?
  • Regression tests added?
  • ChangeLog updates (for next release)
  • Documentation added?
• Test changes
  • Checked in changed Readme.adoc (make test-spec)
  • Added new test to group Readme.adoc and yaml file
• Code style update (formatting, renaming)
• Refactoring (please detail in commit messages)
• Build related changes
• Documentation content changes
  • ChangeLog updated (for major changes)
• Other (please describe):

Disclaimer: Author: Peter Woxblom. Drafting and implementation assisted
by Jarvis, an AI agent (Hermes on DeepSeek V4 Flash backend). Reviewed
and tested on hardware by the author.