Since commit e69f5dc6 ("dt-bindings: serial: Convert 8250 to
json-schema"), the schema for "ns16550a" is checked.

Since then, 'make ARCH=arm64 dtbs_check' is so noisy because the
required property 'interrupts' is missing.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>