arm: dts: am654-base-board-uboot: Add aliases for USB subsystems

The sequence number assigned for USB subsystem in a uclass is dependent on
the order of occurrence in the device tree. If the dr_mode of USB3SS0
controller is varied then the sequence number of USB3SS1 controller also
changes. If aliases are added then sequence numbers are assigned using the
alias number. This makes the sequence number of USB3SS1 controller
independent of USB3SS0 controller's dr_mode.

Therefore, add aliases to fix the sequence number assigned to the USB
subsystems.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>