platform/x86: amd-pmc: Add AMD platform support for S2Idle

platform/x86: amd-pmc: Add AMD platform support for S2Idle

AMD Power Management Controller driver a.k.a. amd-pmc driver is the
controller which is meant for the final S2Idle transaction that goes to
the PMFW running on the AMD SMU (System Management Unit) responsible for
tuning of the VDD.

Once all the monitored list or the idle constraints are met, this driver
would go and set the OS_HINT (meaning all the devices have reached to
their lowest state possible) via the SMU mailboxes.

This driver would also provide some debug capabilities via debugfs.

Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20201105140531.2955555-1-Shyam-sundar.S-k@amd.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>