ARM: dts: am57xx-beagle-x15: Switch GPIO fan number

BeagleBoard-X15 pre-production change includes switching the GPIO fan
gpio over from 1 to 2 to allow for a potential fix at a later point in
time for USB client VBUS detection using PMIC VBUS detect capability.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>