The return value '-ENOSPC' of fit_set_timestamp function does not match
the caller fit_image_write_sig's expection which is '-FDT_ERR_NOSPACE'.

Fix it by not calling fit_set_timestamp, but call fdt_setprop instead.

This fixes a following mkimage error:
| Can't write signature for 'signature@1' signature node in
| 'conf@imx6ull-colibri-wifi-eval-v3.dtb' conf node: <unknown error>
| mkimage Can't add hashes to FIT blob: -1

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Tested-by: Igor Opaniuk <igor.opaniuk@foundries.io>