Add deserialization of AccumType

Added deserialize accum_type attribute for AvgPool2D Op
also fixed serialization of accum_type attribute for AvgPool2D Op

also updated third_party/serialization_lib hash

LLVM_REFSPEC:  refs/changes/23/532123/2
TF_REFSPEC: refs/changes/34/699334/5

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: I2084f33e60d1bf8f76958b320a96fc1f3a94d95c