ARM: dts: bcm2835-rpi-zero-w: Fix bus-width of sdhci

The commit e7774049 ("ARM: dts: bcm283x: Define MMC interfaces at
board level") accidently dropped the bus width for the sdhci on the
RPi Zero W, because the board file was relying on the defaults
from bcm2835-rpi.dtsi. So fix this performance regression by adding
the bus width to the board file.

Fixes: e7774049 ("ARM: dts: bcm283x: Define MMC interfaces at board level")
Reported-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>