pinctrl: mcp23s08: spi: Fix duplicate pinctrl debugfs entries

This is a bit more involved because the pinctrl core so far always
assumed that one device (with a unique dev_name) only contains a single
pinctrl thing. This is not true for the mcp23s08 driver for chips
connected over SPI. They have a "logical address" which means that
several chips can share one physical CS signal.

A downside of this patch are some possibly ugly names for the debugfs
entries, such as "spi1.1-mcp23xxx-pinctrl.2", etc.

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