thermal: Add PCIe cooling driver

Add a thermal cooling driver to provide path to access PCIe bandwidth
controller using the usual thermal interfaces.

A cooling device is instantiated for controllable PCIe Ports from the
bwctrl service driver.

If registering the cooling device fails, allow bwctrl's probe to succeed
regardless. As cdev in that case contains IS_ERR() pseudo "pointer", clean
that up inside the probe function so the remove side doesn't need to
suddenly make an odd looking IS_ERR() check.

The thermal side state 0 means no throttling, i.e., maximum supported PCIe
Link Speed.

Link: https://lore.kernel.org/r/20241018144755.7875-9-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
[bhelgaas: dropped data->cdev test per
https://lore.kernel.org/r/ZzRm1SJTwEMRsAr8@wunner.de

]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org> # From the cooling device interface perspective