ARM: dts: kirkwood: fix SD slot default configuration for OpenRD

The SD card slot was enabled by default with legacy booting.
It does not work anymore with DT boot. Fix by providing GPIO configuration
that matches the old default.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>