[terraform-ibm-icd-postgresql] Scope KMS policy to the exact KMS key

[ocofaigh]

The policy here can be updated to scope it to the exact KMS key. For an example of the syntax, see terraform-ibm-modules/terraform-ibm-cos#764

[ocofaigh]

This work will be included as part of the effort in #541 which Jordan has already started on, so re-assigning to him