mfd: da9063: Add support for OnKey driver

Add MFD support for the DA9063 OnKey driver

The function da9063_clear_fault_log() is added to mitigate the case of a
hardware power-cut after a long-long OnKey press. Although there is no
software intervention in this case (by definition) such a shutdown would
cause persistent information within the DA9063 FAULT_LOG that would be
available during the next device restart.

Clearance of this persistent register must be completed after such a
hardware power-cut operation has happened so that the FAULT_LOG does not
continue with previous values. The clearance function has been added here
in the kernel driver because wiping the fault-log cannot be counted on
outside the Linux kernel.

Signed-off-by: Steve Twiss <stwiss.opensource@diasemi.com>
[Lee: Removed 'key_power' for Dmitry to take through the Input Tree]
Signed-off-by: Lee Jones <lee.jones@linaro.org>