dt-binding: mfd: as3722: Correct macro name

The macro name for enable3 pin is named as AS3722_EXT_CONTROL_PIN_ENABLE2
which is conflict with the enable2 pin.

Correct this macro name to correctly reflect the enable pin i.e.
AS3722_EXT_CONTROL_PIN_ENABLE3.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reported-by: Dan Willemsen <dwillemsen@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>