dt-bindings: serial: Convert arm,pl011 to json-schema

Convert the arm,pl011 binding to DT schema using json-schema.

The zte,zx296702-uart binding appears to be broken as the dts files are
missing 'arm,primecell'. That's included in the schema here to throw a
warning.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>