mfd: twl6040: Constify struct regmap_config and reg_default array

The regmap_config struct may be const because it is not modified by the
driver and regmap_init() accepts pointer to const. Make array of
struct reg_default const as well.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>