Allocate and free page directories on demand.

V2:
a. clear entries allocation
b. fix entries index calculation
c. need alloc sub level even parent bo was allocated

Signed-off-by: Christian König <christian.koenig@amd.com> (v1)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Signed-off-by: Chunming Zhou <David1.Zhou@amd.com> (v2)
Acked-by: Alex Deucher <alexander.deucher@amd.com> (v2)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>