pinctrl: mcp23s08: spi: Fix regmap allocation for mcp23s18

Fixes issue created by 9b3e4207

.

It wasn't possible for one_regmap_config to be non-NULL at the point
it was tested for mcp23s18 devices.

Applied the same pattern of allocating one_regmap_config using
devm_kmemdump() and then initializing the local regmap structure
from that.

Signed-off-by: Jason Kridner <jdk@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>