mfd: syscon: Add platform data with a regmap config name

Define syscon platform data structure that can be used
to define a regmap config name. This is particularly useful
in the regmap debugfs when there is more than one syscon
device registered, to distinguish the register blocks.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Acked-by: Lee Jones <lee.jones@linaro.org>