powerpc/cpu/mpc85xx: Add MAC address for layer 2 switch

T1040RDB and T1040QDS boards have an integrated l2 switch.
The switch needs a MAC address for Layer 2 protocols
(MSTP, LLDP, LACP, etc). Setting a MAC address on l2switchaddr will add
a MAC in device-tree, under node l2switch.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@freescale.com>
Cc: York Sun <yorksun@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>