Documentation: DT: Add entry for FSL Management Complex

This patch adds a devicetree binding documentation for FSL's
Management Complex.

Management Complex is a hardware resource manager that manages
specialized hardware objects used in network-oriented packet
processing applications

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>
Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
Signed-off-by: J. German Rivera <German.Rivera@freescale.com>
Signed-off-by: Olof Johansson <olof@lixom.net>