mfd: syscon: Allow property as NULL in syscon_regmap_lookup_by_phandle

If we pass syscon device node itself as first parameter to this function
there is no need to parse and find syscon device node. So by allowing
"property" parameter as NULL allow syscon_regmap_lookup_by_phandle to
consider passed argument "np" itself as a syscon device node "syscon_np".
This will help us in avoiding addition of "syscon" property in a device node
which points back to same device node in device tree.

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>