If simple multifuction device also provides syscon interface, its
childern should be able to consume it. Due to this:
- declare coresponding method in syscon interface
- implement it in simple multifunction device driver
MFC after: 1 week
that node is also compatible with syscon. For instance,
Rockchip RK3399's GRF (General Register Files) is compatible
with simple-mfd as well as syscon and has devices like
usb2-phy, emmc-phy and pcie-phy etc. under it.
Reviewed by: manu
