ARM: dts: am57xx-beagle-x15: Provide supply for usb2_phy2

Without this USB2 breaks if USB1 is disabled or USB1
initializes after USB2 e.g. due to deferred probing.

Fixes: 5a0f93c6

 ("ARM: dts: Add am57xx-beagle-x15")
Signed-off-by: Roger Quadros <rogerq@ti.com>
Cc: stable@vger.kernel.org (v3.19+)
Signed-off-by: Tony Lindgren <tony@atomide.com>