BUG: Allow freq conversion from dt64 to period
Closes #23438

In the PeriodArray refactor, I disallowed changing freq when converting from datetime64 to period. I didn't realize that was supported.

cc @thequackdaddy, does this close all the issues you ran into?
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23460?src=pr&el=h1) Report
> Merging [#23460](https://codecov.io/gh/pandas-dev/pandas/pull/23460?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4f717551b8e49ecd978bb4f5a985d6f6a96d86e9?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23460/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23460?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23460      +/-   ##
==========================================
+ Coverage   92.22%   92.23%   +<.01%     
==========================================
  Files         161      161              
  Lines       51187    51179       -8     
==========================================
- Hits        47209    47204       -5     
+ Misses       3978     3975       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <100%> (ø)` | :arrow_up: |
| #single | `42.26% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23460?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/23460/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `98.08% <100%> (+0.59%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23460?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23460?src=pr&el=footer). Last update [4f71755...f765d21](https://codecov.io/gh/pandas-dev/pandas/pull/23460?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @TomAugspurger 

@jbrockmendel if you want to make an issue / note about the de-duplication would be great.