gpio: mcp23s08: convert driver to DT

This converts the mcp23s08 driver to be able to be used with
device tree.

There is a "spi-present-mask" device tree property, that allows to
use multiple of this spi chips on the same chipselect.

v4:
- removed the ability to specify the pullup from device tree
- updated binding doc

v3:
- removed mcp,chips device tree property in favour of a
    mcp,spi-present-mask and a flag for the pullup of every gpio
- seperated the match table. Now there is one for i2c and one for spi
- do the of reading stuff on stack of the probe function - no devm
    any more

v2:
- squashed booth patches together
- fixed build warning, when CONFIG_OF is not defined
- use of_match_ptr macro for of_match_table

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>