mfd: da9150: Use relative paths in DT bindings document

When referencing other DT bindings documentation, use relative
path rather than absolute.
Suggested-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NAdam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/da9150.txt

@@ -17,13 +17,13 @@ Required properties:
   the IRQs from da9150 are delivered to.
 - interrupts: IRQ line info for da9150 chip.
 - interrupt-controller: da9150 has internal IRQs (own IRQ domain).
-  (See Documentation/devicetree/bindings/interrupt-controller/interrupts.txt for
+  (See ../interrupt-controller/interrupts.txt for
    further information relating to interrupt properties)
 
 Sub-devices:
-- da9150-gpadc: See Documentation/devicetree/bindings/iio/adc/da9150-gpadc.txt
-- da9150-charger: See Documentation/devicetree/bindings/power/da9150-charger.txt
-- da9150-fg: See Documentation/devicetree/bindings/power/da9150-fg.txt
+- da9150-gpadc: See ../iio/adc/da9150-gpadc.txt
+- da9150-charger: See ../power/da9150-charger.txt
+- da9150-fg: See ../power/da9150-fg.txt
 
 Example: