qoriq/p1_p2_rdb: 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>