msm8974: Enable SDHCI HS400 mode for MSM8974.

Use target specific config option to enable HS400 mode.

Change-Id: I67812df1485eb8619c25f1369cb8f08d49d3e574
diff --git a/target/msm8974/init.c b/target/msm8974/init.c
index 3dea314..9f081f7 100644
--- a/target/msm8974/init.c
+++ b/target/msm8974/init.c
@@ -229,6 +229,7 @@
 	config.sdhc_base = mmc_sdhci_base[config.slot - 1];
 	config.pwrctl_base = mmc_sdc_base[config.slot - 1];
 	config.pwr_irq     = mmc_sdc_pwrctl_irq[config.slot - 1];
+	config.hs400_support = 1;
 
 	if (!(dev = mmc_init(&config))) {
 		/* Trying Slot 2 next */