Install example psu-update@.service

The repo needs an example psu-update@.service, it was expected to be
installed by a bitbake recipe, but during the recipe review it turns
out that the repo is bet

Install example psu-update@.service

The repo needs an example psu-update@.service, it was expected to be
installed by a bitbake recipe, but during the recipe review it turns
out that the repo is better to install a default one and it could be
overriden by a bbappend.

Move the psu-update@.service from vendor-example to services directory
and install it.

Tested: Verify the service is installed.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: I105b201972b32ca3d9e14878b92c9d038a41cbdb

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