gpio: mcp23s08: Add support for mcp23s18

This patch adds support for the mcp23s18 which is very similar to
the mcp23s17. A couple of control bits are not the same.
Notable IOCON_HAEN (s17 only) & IOCON_INTCC. Which can be ignored.

Patch changes the following:
- Add mcp23s18 types.
- Always set mirror bit if the dts defines mcp23s18. regardless of type.
  Mirror bit is ignored on 8 bit devices anyway.
- In mcp23s08_probe use chip.ngpio instead of logic based on type
  to determine number of gpio lins to increment by. This is set
  appropiately by the call to mcp23s08_probe_one.
- Add mcp23s18 to device tree documentation.
- Remove statement that irqs don't work for spi. They do.
  Tested with mcp23s18.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>