firefly: add interrupt to detect VBUS transitions quickly
firefly: add interrupt to detect VBUS transitions quickly

Use the interrupt as faster detection when VBUS is going off, so we are
not missing when the source is cutting its output due to a fault.

Signed-off-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

BRANCH=none
BUG=none
TEST=Connect Firefly to Zinger, trigger a short OCP with the electronic
load, once Zinger has recovered from the fault, see Firefly
re-negotiating voltage.

Change-Id: I4d273a0007d1e79884e0acbf75509ab9c8578893
Reviewed-on: https://chromium-review.googlesource.com/207031
Tested-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Commit-Queue: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

akappy7/ChromeOS_EC_LED_Diagnostics,longsleep/ec,thehobn/ec,eatbyte/chromium-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,thehobn/ec,md5555/ec,fourier49/BIZ_EC,thehobn/ec,coreboot/chrome-ec,eatbyte/chromium-ec,fourier49/BIZ_EC,akappy7/ChromeOS_EC_LED_Diagnostics,eatbyte/chromium-ec,fourier49/BZ_DEV_EC,md5555/ec,longsleep/ec,md5555/ec,mtk09422/chromiumos-platform-ec,mtk09422/chromiumos-platform-ec,longsleep/ec,fourier49/BZ_DEV_EC,coreboot/chrome-ec,mtk09422/chromiumos-platform-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,coreboot/chrome-ec,coreboot/chrome-ec,fourier49/BIZ_EC,thehobn/ec,eatbyte/chromium-ec,akappy7/ChromeOS_EC_LED_Diagnostics,longsleep/ec,fourier49/BZ_DEV_EC,fourier49/BIZ_EC,mtk09422/chromiumos-platform-ec,md5555/ec