Add PHY support for Kintex/Zynq Ultrascale+ GTH transceiver

[rniwase]

This pull request includes the following updates:

• Implemented GTH transceivers (GTHE4_CHANNEL) for Kintex/Zynq Ultrascale+ in LiteSATAPHY.
• Verified operation at SATA Gen.3 on GTH transceivers for Kintex Ultrascale+ (XCKU15P) or Zynq Ultrascale+ (XCZU9).
• For Ultrascale+ series GTH/GTY transceivers, when supplying REFCLK via BUFG, it needs to be connected to GTGREFCLK. If supplied from the MGTREFCLK pin, it should be connected to GTREFCLK0. To allow users to explicitly switch, the argument "use_gtgrefclk" has been added to USPLiteSATAPHY or GTHE4LiteSATAPHY.
• When selecting Virtex/Kintex/Zynq Ultrascale+ devices in LiteSATAPHY, an argument "gt_type" has been added to allow users to choose between GTH (GTHE4_CHANNEL) and GTY (GTYE4_CHANNEL).
• Added a bench target for ZCU102 (zcu102.py) as a design example.

This update maintains compatibility with previous designs.

[rniwase]

I felt there was insufficient explanation in this pull request, so I'll leave a comment.

Currently in LiteSATA, it seems possible to implement PHY exclusively for Ultrascale series GTH transceivers using USLiteSATAPHY, or alternatively, using USPLiteSATAPHY for Ultrascale+ series GTY transceivers.

Despite Ultrascale having GTY (GTYE3_CHANNEL) and Ultrascale+ having GTH (GTHE4_CHANNEL), I've unfortunately noticed the absence of their implementation in LiteSATAPHY. (Thus, the content in the README appears to be inaccurate.)

For my project's immediate needs, requiring the use of Kintex Ultrascale+ GTH (GTHE4_CHANNEL), I've newly implemented the PHY (GTHE4LiteSATAPHY) this time.

While the file and module names were arbitrarily chosen, it might be clearer to rename USLiteSATAPHY/USPLiteSATAPHY to something more accurate (for example, following the pattern of liteiclink/serdes) especially when considering potential future implementations of new PHYs like GTYE3_CHANNEL.

I'd appreciate a review regarding this matter.

[enjoy-digital]

Thanks a lot @rniwase! (and sorry for the delay). This is a very nice contribution!

USLiteSATAPHY/USPLiteSATAPHY naming is indeed not clear and I also realized and changed recently in LiteICLink. I just created https://github.com/enjoy-digital/litesata/issues/32 to look at it and will try to do it soon.

We can already merge your PR. Thanks again!