gpio: mcp23s08: rename the device tree property

The device tree property should be more descriptive.
microchip seems more reasonable than mcp. The old mcp
prefix is still supported but marked as deprecated.
Users of mcp have to switch to the microchip prefix.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>