u-boot-tools: fix a mkimage signature issue

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

This is caused by a wrong return value being used in uboot source.

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.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>