checks: fix handling of unresolved phandles for dts plugins

In dts plugins, it is valid to have unresolved phandle values. The
check_property_phandle_args and check_interrupts_property checks failed to
account for this resulting in spurious warnings or asserts, respectively.
Fix this by bailing from the checks if we're checking a dts plugin as
there is no way to further validate the properties.

Fixes: ee3d26f6 ("checks: add interrupts property check")
Fixes: b3bbac02 ("checks: add phandle with arg property checks")
Reported-by: Alan Tull <atull@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>