cpufreq/amd-pstate: drop stale @epp_cached kdoc

cpufreq/amd-pstate: drop stale @epp_cached kdoc

Commit 4e16c1175238 ("cpufreq/amd-pstate: Stop caching EPP") removed
the epp_cached field from struct amd_cpudata in favour of always
reading from cppc_req_cached, but the kdoc above the struct still
documents @epp_cached.

Drop the now-stale @epp_cached entry.

Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Fixes: 4e16c1175238 ("cpufreq/amd-pstate: Stop caching EPP")
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Link: https://lore.kernel.org/r/20260526022131.1302373-1-zhanxusheng@xiaomi.com
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>