arm: mvebu: mcbin: dts: fix PCIe reset polarity

The PCIe slot PERST signal is active low. Fix the gpio signal
description in the dts.

This happened to work because the pcie_dw_mvebu driver sets the reset
gpio level to 1 (high) to release the reset. The following commit will
fix that.

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