mfd: da9062: Support for the DA9063 OnKey in the DA9062 core

Add MFD core driver support for a OnKey component

- MFD core adds the resource da9062_onkey_resources[] for the OnKey
- An appropriate value has been added into mfd_cell da9062_devs[] to
  support component .name = "da9062-onkey" and
  .of_compatible  = "dlg,da9062-onkey"

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