arm: zynq: Use s25fl256s1 compatible string on zedboard

Use compatible string which is listed in the Linux kernel.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/49bc6b056b0f6f69d4d90351dc875a66b7e37619.1631715748.git.michal.simek@xilinx.com