pinctrl: mcp23s08: spi: Fix regmap debugfs entries

The SPI version of this chip allows several devices to be present on the
same SPI bus via a local address. If this is in action and if the kernel
has debugfs, however, the code attempts to create duplicate entries for
the regmap's debugfs:

  mcp23s08 spi1.1: Failed to create debugfs directory

This patch simply assigns a local name matching the device logical
address to the `struct regmap_config`.

No changes are needed for MCP23S18 because that device does not support
any logical addressing. Similarly, I2C devices do not need any action,
either, because they are already different in their I2C address.

A similar problem is present for the pinctrl debugfs instance, but that
one is not addressed by this patch.

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