powerpc/85xx: Fix P1020RDB boot hang due USB2

Since USB2 is shared with local bus, either local bus or USB2
should be disabled. By default U-Boot enables local bus, so we
have to disable USB2, otherwise kernel hangs:

 ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
 fsl-ehci fsl-ehci.0: Freescale On-Chip EHCI Host Controller
 fsl-ehci fsl-ehci.0: new USB bus registered, assigned bus number 1
 fsl-ehci fsl-ehci.0: irq 28, io base 0xffe22000
 fsl-ehci fsl-ehci.0: USB 2.0 started, EHCI 1.00
 hub 1-0:1.0: USB hub found
 hub 1-0:1.0: 1 port detected
 fsl-ehci fsl-ehci.1: Freescale On-Chip EHCI Host Controller
 fsl-ehci fsl-ehci.1: new USB bus registered, assigned bus number 2
 <hangs here>

Note that U-Boot doesn't clear 'status' property when it enables
USB2, so we have to comment out the whole node.

To enable USB2, one can issue
'setenv hwconfig usb2:dr_mode=<host|peripheral>' command at the
U-Boot prompt.

Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>