arm: mvebu: armada-xp-theadorable.dts: Change CS# for 2nd FPGA

The new board version has the 2nd FPGA connected via CS# 0 instead of
2 on SPI bus 1. Change this setup in the DT accordingly. Please note
that this change does still work on the old board version because the
CS signal is not used on this board.

Signed-off-by: Stefan Roese <sr@denx.de>