pinctrl: mcp23s08: Add optional reset GPIO

The MCP23x port expander RESET# line can be connected to a host GPIO.
The optional reset-gpio must be set to LOW if the reset is asserted
at probing time.

On page 5 in the datasheet [0] the "Device Active After Reset high"
time is specified at 0 µs. Therefore no waiting is needed after the
reset transition.

[0] https://ww1.microchip.com/downloads/en/DeviceDoc/20001952C.pdf

Signed-off-by: Andreas Kaessens <akaessens@gmail.com>
Signed-off-by: Darian Biastoch <d.biastoch@gmail.com>
Link: https://lore.kernel.org/r/20210610132438.3085841-1-akaessens@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>