ARM: bcm2835-rpi-zero-w: Add missing pinctrl name

Define the sdhci pinctrl state as "default" so it gets applied
correctly and to match all other RPis.

Fixes: 2c7c040c

 ("ARM: dts: bcm2835: Add Raspberry Pi Zero W")
Signed-off-by: Nick Hudson <skrll@netbsd.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>