MAINTAINERS: Add Dialog PMIC search terms for missing documentation and header files

Dialog Semiconductor support would like to follow files by adding to the
existing MAINTAINERS search terms. The update will allow us to follow
files for PMIC documentation bindings and header files.

The full list is:

DT bindings
 - Documentation/devicetree/bindings/mfd/da9052-i2c.txt
 - Documentation/devicetree/bindings/mfd/da9055.txt
 - Documentation/devicetree/bindings/mfd/da9062.txt
 - Documentation/devicetree/bindings/mfd/da9063.txt
Header files
 - include/linux/mfd/da9062/core.h
 - include/linux/mfd/da9062/registers.h

Signed-off-by: Steve Twiss <stwiss.opensource@diasemi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>