arm: boot: beaglex15: pass correct interrupt

According to latest schematics [1], GPIO_1/VBUSDET
on TPS659038 is tied to AM57x GPIO4_21. We can use
that as a VBUS interrupt, instead of relying on
PMIC's VBUS interrupts which don't seem to be firing
on x15 at all.

A follow up patch will add support for using this
GPIO-based interrupt mechanism for notifying about
VBUS.

[1] https://github.com/beagleboard/beagleboard-x15/blob/master/BeagleBoard-X15_RevA2.pdf



Signed-off-by: Felipe Balbi <balbi@ti.com>
[cw00.choi: Use the 'vbus-gpio' property instead of 'interrupts-extended']
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Tony Lindgren <tony@atomide.com>