mfd: 88pm860x: Avoid to check resource for preg regulator

Since PREG regulator is the only one regulator in 88PM8606, and other
regulators are in 88PM8607. Checking resource as identifying regulator
is not a good way. We can use NULL resource to indentify PREG regulator.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>