Add CEL to the arch field in NodePool to prevent error when the cloud platform isn't a supported one

User Story:

As a user of multi-arch HyperShift, I would like a CEL validation to be added to the NodePool types to prevent the arch field from being changed from `amd64` when the platform is not supported (AWS is currently the only supported platform).

Acceptance Criteria:

Description of criteria:

• CEL added to the arch field for NodePools which will error if a user tries to change the arch field from `amd64` for any other platform other than AWS.

(optional) Out of Scope:

Detail about what is specifically not being delivered in the story

Engineering Details:

• (optional) https://github/com/link.to.enhancement/
• (optional) https://issues.redhat.com/link.to.spike
• Engineering detail 1
• Engineering detail 2

This requires/does not require a design proposal.
This requires/does not require a feature gate.