imx8mm_beacon: Enable fixed regulator in SPL

Because SPL can support SD UHS, the fixed regulator needs to be
enabled in SPL to reset the SD card.

Fixes: 1a5d9c84b472 ("imx8mm_beacon: Enable HS400 on MMC controller")
Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/imx8mm_beacon_defconfig b/configs/imx8mm_beacon_defconfig
index 49d5453..b0ea87a 100644
--- a/configs/imx8mm_beacon_defconfig
+++ b/configs/imx8mm_beacon_defconfig
@@ -95,6 +95,7 @@
 CONFIG_DM_REGULATOR_BD71837=y
 CONFIG_SPL_DM_REGULATOR_BD71837=y
 CONFIG_DM_REGULATOR_FIXED=y
+CONFIG_SPL_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_CONS_INDEX=2
 CONFIG_DM_SERIAL=y