arm: mvebu: cf gt-8k: dts: add PCIe slot reset support

Describe the mini-PCIe slot gpio reset signal. This enables PCIe devices
on Clearfog GT-8K.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>