MFC r349596 by ganbold:

Extend simple_mfd driver to expose a syscon interface if
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