dt-bindings: sifive-l2-cache: Fix 'select' matching

When the schema fixups are applied to 'select' the result is a single
entry is required for a match, but that will never match as there should
be 2 entries. Also, a 'select' schema should have the widest possible
match, so use 'contains' which matches the compatible string(s) in any
position and not just the first position.

Fixes: 993dcfac ("dt-bindings: riscv: sifive-l2-cache: convert bindings to json-schema")
Signed-off-by: Rob Herring <robh@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>