net: dsa: mt7530: MT7530 optional GPIO support

MT7530's LED controller can drive up to 15 LED/GPIOs.

Add support for GPIO control and allow users to use its GPIOs by
setting gpio-controller property in device tree.

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>