ARM: zynq: Fix compatible string for adi,adxl345 chip

The commit e359a292 ("dt-bindings: iio: accel: adxl345: switch to YAML
bindings") switched binding to yaml and the following error pop up:
../zynq-zturn-v5.dt.yaml: accelerometer@53: compatible: 'oneOf' conditional
failed, one must be
fixed:
['adi,adxl345', 'adxl345', 'adi,adxl34x', 'adxl34x'] is too long
Additional items are not allowed ('adi,adxl34x', 'adxl34x' were unexpected)
Additional items are not allowed ('adxl345', 'adi,adxl34x', 'adxl34x' were
unexpected)
'adi,adxl346' was expected
'adi,adxl345' was expected

Use only one compatible string to be aligned with the binding.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/a9075ab54df13461380e46d3002302d3672325b5.1606397101.git.michal.simek@xilinx.com