gpio: mcp23s08 to support both device tree and platform data
Device tree is not enabled in some architecture where gpio
driver mcp23s08 is still required.
v2-changes:
- Parse device tree properties into platform data other than
individual variables.
v3-changes:
- Use of_node in gpio_chip device structure, because the
struct device * always has an of_node which is NULL when
OF is not used.