icu: update icudata generator URL

It was pointing to version 51.x which wouldn't work well with 55.x.

Point it to the latest version that matches the current buildroot
version (55). When a new ICU major version is released a link for the
"old" 55 version will be available below as is it currently for
previous versions such as 54, 53 and so on.

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>