Implement hwmon API for reading/setting slow and fast PPT limit.

APU power is managed to system-level requirements through the PPT
(package power tracking) feature. PPT is intended to limit power to the
requirements of the power source and could be dynamically updated to
maximize APU performance within the system power budget.

Here FAST_PPT_LIMIT manages the ~10 ms moving average of APU power,
while SLOW_PPT_LIMIT manages the configurable, thermally significant
moving average of APU power (default ~5000 ms).

User could read slow/fast ppt limit using command "cat power*_cap" or
"sensors" in the hwmon device directory. User could adjust values of
slow/fast ppt limit as needed depending on workloads through command
"echo ## > power*_cap".

Example:
$ echo 15000000 > power1_cap
$ echo 18000000 > power2_cap
$ sensors
amdgpu-pci-0300
Adapter: PCI adapter
slowPPT:     9.04W (cap = 15.00 W)
fastPPT:     9.04W (cap = 18.00 W)

v2: align with existing interfaces for the getting/setting of PPT
    limits. Encode the upper 8 bits of limit value to distinguish
    slow and fast power limit type.
Signed-off-by: Xiaomeng Hou <Xiaomeng.Hou@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>