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

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