Change ppc64le to handle being passed whole device

[cgwalters]

Basically the idea here is that bootupd should be passed a whole device, and it should find the relevant partitions inside it on its own. That's true on every architecture right now except ppc64le, where e.g. the CoreOS pipeline only passes the PReP partition explicitly:
https://github.com/coreos/coreos-assembler/blob/main/src/osbuild-manifests/coreos.osbuild.ppc64le.mpp.yaml#L670

And in fact, we worked around this in bootc too https://github.com/containers/bootc/blob/c8e1fb83fe87dc35cbf56773fedc4b60890f390c/lib/src/bootloader.rs#L25

Let's change this project to find the PReP partition itself, then both of those callers can just pass the whole device.

We already have the code of course to find the PReP partition for updates...or actually we did before we merged this PR: https://github.com/coreos/bootupd/pull/824/files#diff-bc0489e73cacb2333386dc9198a92cf42c65a3e389582345835a6619e37b4084L59

@HuijingHei I think we regressed ppc64le with that? I'm not totally sure...

Anyways TL;DR let's be sure our install flow detects when it's passed a whole device and finds the PReP partition itself.

xref https://issues.redhat.com/browse/RHEL-64736

[HuijingHei]

Sorry for the late reply, this is introduced by 07c3661 which ignore the ppc64le platform.