mfd: 88pm860x: Use REG resource for backlight

Now resource of 88pm860x backlight is changed from IORESOURCE_IO
to IORESOURCE_REG. In original driver, the resource is using
self-defined IORESOURCE_IO. So change the resource to register
offset to match the definition of IORESOURCE_REG.

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