sandbox_vpl: Enable missing TPL_DM_I2C symbol

Currently this platform implicity builds CONFIG_TPL_DM_I2C support
without setting the symbol. Add it for clarity.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/sandbox_vpl_defconfig b/configs/sandbox_vpl_defconfig
index 4632939..e1a0555 100644
--- a/configs/sandbox_vpl_defconfig
+++ b/configs/sandbox_vpl_defconfig
@@ -149,6 +149,7 @@
 CONFIG_GPIO_HOG=y
 CONFIG_QCOM_PMIC_GPIO=y
 CONFIG_SANDBOX_GPIO=y
+CONFIG_TPL_DM_I2C=y
 CONFIG_I2C_CROS_EC_TUNNEL=y
 CONFIG_I2C_CROS_EC_LDO=y
 CONFIG_DM_I2C_GPIO=y