config: Add PINCTRL_MCP23S08

As of Linux 4.12, the mcp23s08 driver moved from drivers/gpio to
drivers/pinctrl. At the same time, the Kconfig symbols changed
from CONFIG_GPIO_MCP23S08 to CONFIG_PINCTRL_MCP23S08, effectively
removing the MCP23S17 from the downstream defconfigs. Restore
support by adding CONFIG_PINCTRL_MCP23S08.

See: https://github.com/raspberrypi/linux/issues/2311



Signed-off-by: Phil Elwell <phil@raspberrypi.org>