rng-tools: add systemd-udev-settle wants to service

rngd needs to start after `systemd-udev-settle` in order for the kernel
modules of the random source hardware to be loaded before it is started.

However, since the `rngd.service` does not require or want
`systemd-udev-settle.service` it might not be scheduled for start and
the `After=systemd-udev-settle.service` there has no effect.

Adding `Wants=systemd-udev-settle.service` provides a weak requirement
to it, so that the `rngd` is started after it, if possible.

Signed-off-by: Claudius Heine <ch@denx.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit e9715d42)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>