dt-bindings: gpio: sifive: add gpio-line-names

dt-bindings: gpio: sifive: add gpio-line-names

Fix device tree schema validation messages like 'gpio-line-names'
does not match any of the regexes: 'pinctrl-[0-9]+' From schema: ...
sifive,gpio.yaml'.

The bindings were missing the gpio-line-names element, which was
causing the dt-schema checker to trip-up.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Atul Khare <atulkhare@rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220803155539.800766-1-mail@conchuod.ie
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>