powerpc/usb: Workaround for erratum-A006261

USB spec says that the minimum disconnect threshold should be
	over 525 mV. However, internal USB PHY threshold value is below
	this specified value. Due to this some devices disconnect at
	run-time. Hence, phy settings are tweaked to increased disconnect
	threshold to be above 525mV by using this workaround.

Signed-off-by: Suresh Gupta <suresh.gupta@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>