mmc: sdhci-iproc: Add support for emmc2 of the BCM2711

The additional emmc2 interface of the BCM2711 is an improved version
of the old emmc controller, which is able to provide DDR50 mode on the
Raspberry Pi 4. Except 32 bit only register access no other quirks are
known yet.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>