mfd: wm5110: Expose DRE control registers

Certain use-cases require the DRE to be disabled so expose registers
necessary to control the DRE enables.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>