dt-bindings: mmc: sdhci-iproc: Add brcm,bcm2711-emmc2

Add a new compatible for the additional emmc2 controller
on BCM2711 and clearify usage.

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