USB vbus 5V is from PMIC SWBST, so set swbst_reg as vbus's
parent reg, it fixed a bug that the voltage of vbus is incorrect
due to swbst_reg is disabled after boots up.

Cc: stable@vger.kernel.org
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>