doc: add power uncore in API index

Add missing power uncore API documentation.

Fixes: ac1edcb ("power: refactor uncore power management API")
Cc: stable@dpdk.org

Reported-by: Vipin Varghese <vipin.varghese@amd.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@amd.com>

doc/api/doxy-api-index.md

@@ -103,6 +103,7 @@ The public API headers are grouped by topics:
   [service cores](@ref rte_service.h),
   [keepalive](@ref rte_keepalive.h),
   [power/freq](@ref rte_power.h),
+  [power/uncore](@ref rte_power_uncore.h),
   [PMD power](@ref rte_power_pmd_mgmt.h)
 
 - **layers**: