mfd: Always assign of_node in mfd_add_device()

mfd_add_device() assigns .of_node in the device objects it creates only
if the mfd_cell for the device has the .of_compatible field set and the
DT node for the top-level MFD device contains a child whose compatible
property matches the cell's .of_compatible field.

This leaves .of_node unset in many cases. When this happens, entries in
the DT /aliases property which refer to the top-level MFD DT node will
never match the MFD child devices, hence causing the requested alias not
to be honored.

Solve this by setting each MFD child device's .of_node equal to the top-
level MFD device's .of_node field in the cases where it would otherwise
remain unset.

The first use-case for this will be aliases for the TPS6586x's RTC
device.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>