Documentation/amd-pstate: List amd_pstate_hw_prefcore sysfs file

Documentation/amd-pstate: List amd_pstate_hw_prefcore sysfs file

Add the missing amd_pstate_hw_prefcore filenames in the sysfs listing
example leading to the descriptions of these parameters. Clarify when
will the file be visible.

Fixes: b96b82d1af7f ("cpufreq: amd-pstate: Add documentation for `amd_pstate_hw_prefcore`")
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>