imx: mx6sabresd: fix usb start when DM enabled

Enable CONFIG_DM_REGULATOR_FIXED to let vbus regulator work.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/configs/mx6sabresd_defconfig b/configs/mx6sabresd_defconfig
index b2c77ff..93f0ee9 100644
--- a/configs/mx6sabresd_defconfig
+++ b/configs/mx6sabresd_defconfig
@@ -84,6 +84,7 @@
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX6=y
 CONFIG_DM_REGULATOR=y
+CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_SPI=y
 CONFIG_DM_SPI=y
 CONFIG_MXC_SPI=y