ARM: dts: bcm283x: Define MMC interfaces at board level

commit e7774049 upstream.

Starting with RPi 4 this is the first board, which doesn't use sdhost
as default SD interface. So the MMC interfaces should be defined finally at
board level. Since all boards using sdhci already does this, we can drop the
pinctrl part from bcm2835-rpi.dtsi.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Eric Anholt <eric@anholt.net>