zynq-zed.dtsi: Set QSPI flash max frequency to 104MHz

The datasheet
http://www.spansion.com/Support/Datasheets/S25FL128S_256S_00.pdf
specifies 104MHz as the maximum frequency for the "quad read" command.
Reflect this in the devicetree.

Real life performance using 'dd if=/dev/mtd3 of=/dev/null bs=128k' went
from 3.8 to 4.2 MB/s as a result of this change.