qoriq/p1_p2_rdb_pc: USB device-tree fixups for P1020

Resolve P1020 second USB controller multiplexing with eLBC
         - mandatory to mention USB2 in hwconfig string to select it
           over eLBC, otherwise USB2 node is removed
         - works only for SPI and SD boot

Signed-off-by: Ramneek Mehresh <ramneek.mehresh@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Zhicheng Fan <B32736@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>