pinctrl: mcp23s08: spi: Add HW address to gpio_chip.label

When several devices are sharing one hardware SPI CS, there is no visual
clue in `lsgpio` or in /sys/kernel/debug/gpio about which one is which
one. Stuff depends on the enumeration order, and therefore lower chip
addresses always go first, but that's just an implementation detail.
This change includes the device-specific address in the debug output:

  gpiochip4: GPIOs 464-479, parent: spi/spi1.1, mcp23s17.2, can sleep:
  gpiochip3: GPIOs 480-495, parent: spi/spi1.1, mcp23s17.1, can sleep:

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>