mfd: max8907: Remove regulator-compatible from DT docs

Commit "regulator: deprecate regulator-compatible DT property" deprecated
the use of the regulator-compatible DT property. Update the DT example in
the MAX8907 binding documentation to reflect this.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>