mmc: sdhci-iproc: Enable eMMC DDR 3.3V support for bcm2711

The emmc2 interface on the bcm2711 supports DDR modes for eMMC devices
running at 3.3V. This allows to run eMMC module with 3.3V signaling voltage
at DDR52 mode on the Raspberry Pi 4 using a SD adapter:

  clock: 52000000 Hz
  actual clock: 50000000 Hz
  vdd: 21 (3.3 ~ 3.4 V)
  bus mode: 2 (push-pull)
  chip select: 0 (don't care)
  power mode: 2 (on)
  bus width: 2 (4 bits)
  timing spec: 8 (mmc DDR52)
  signal voltage: 0 (3.30 V)
  driver type: 0 (driver type B)

Link: https://github.com/raspberrypi/linux/issues/3802Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/1598651234-29826-1-git-send-email-stefan.wahren@i2se.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/sdhci-iproc.c

@@ -283,6 +283,7 @@ static const struct sdhci_pltfm_data sdhci_bcm2711_pltfm_data = {
 
 static const struct sdhci_iproc_data bcm2711_data = {
 	.pdata = &sdhci_bcm2711_pltfm_data,
+	.mmc_caps = MMC_CAP_3_3V_DDR,
 };
 
 static const struct of_device_id sdhci_iproc_of_match[] = {