phy: spacemit: Introduce PCIe/combo PHY

phy: spacemit: Introduce PCIe/combo PHY

Introduce a driver that supports three PHYs found on the SpacemiT
K1 SoC.  The first PHY is a combo PHY that can be configured for
use for either USB 3 or PCIe.  The other two PHYs support PCIe
only.

All three PHYs must be programmed with an 8 bit receiver termination
value, which must be determined dynamically.  Only the combo PHY is
able to determine this value.  The combo PHY performs a special
calibration step at probe time to discover this, and that value is
used to program each PHY that operates in PCIe mode.  The combo
PHY must therefore be probed before either of the PCIe-only PHYs
will be used.

Each PHY has an internal PLL driven from an external oscillator.
This PLL started when the PHY is first initialized, and stays
on thereafter.

During normal operation, the USB or PCIe driver using the PHY must
ensure (other) clocks and resets are set up properly.

However PCIe mode clocks are enabled and resets are de-asserted
temporarily by this driver to perform the calibration step on the
combo PHY.

Tested-by: Junzhong Pan <panjunzhong@linux.spacemit.com>
Signed-off-by: Alex Elder <elder@riscstar.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/all/ba532f8d-a452-40e5-af46-b58b89f70a92@linaro.org/
Tested-by: Yixun Lan <dlan@gentoo.org>
Link: https://patch.msgid.link/20251218151235.454997-4-elder@riscstar.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>