phy: cadence-torrent: Add support to drive refclk out

phy: cadence-torrent: Add support to drive refclk out

cmn_refclk_<p/m> lines in Torrent SERDES are used for connecting an
external reference clock. cmn_refclk_<p/m> can also be configured to
output the reference clock. Model this derived reference clock as a
"clock" so that platforms like AM642 EVM can enable it.

This is used by PCIe to use the same refclk both in local SERDES
and remote device. Add support here to drive refclk out.

Based on: https://lore.kernel.org/all/20210310120840.16447-7-kishon@ti.com/

Signed-off-by: George McCollister <george.mccollister@konsulko.com>
Tested-by: Bryan Brattlof <bb@ti.com>