sandbox: Enable MC34708 PMIC support

This MC34708 PMIC is somewhat special - it used single transfers (R/W) with
3 bytes size - up till now U-Boot's PMICs only used 1 byte.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index c1cdd59..2fc84a1 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -142,6 +142,7 @@
 CONFIG_PMIC_ACT8846=y
 CONFIG_DM_PMIC_PFUZE100=y
 CONFIG_DM_PMIC_MAX77686=y
+CONFIG_DM_PMIC_MC34708=y
 CONFIG_PMIC_PM8916=y
 CONFIG_PMIC_RK8XX=y
 CONFIG_PMIC_S2MPS11=y