of: Skip kunit tests when arm64+ACPI doesn't populate root node

A root node is required to apply DT overlays. A root node is usually
present after commit 7b937cc2 ("of: Create of_root if no dtb
provided by firmware"), except for on arm64 systems booted with ACPI
tables. In that case, the root node is intentionally not populated
because it would "allow DT devices to be instantiated atop an ACPI base
system"[1].

Introduce an OF function that skips the kunit test if the root node
isn't populated. Limit the test to when both CONFIG_ARM64 and
CONFIG_ACPI are set, because otherwise the lack of a root node is a bug.
Make the function private and take a kunit test parameter so that it
can't be abused to test for the presence of the root node in non-test
code.

Use this function to skip tests that require the root node. Currently
that's the DT tests and any tests that apply overlays.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/r/6cd337fb-38f0-41cb-b942-5844b84433db@roeck-us.net
Link: https://lore.kernel.org/r/Zd4dQpHO7em1ji67@FVFF77S0Q05N.cambridge.arm.com

 [1]
Fixes: 893ecc6d ("of: Add KUnit test to confirm DTB is loaded")
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20241009204133.1169931-1-sboyd@kernel.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>