[ref model] fix const/pad/clamp attribute serialization

This changes to use native type serialization and deserialization
for pad_const, clamp min_val/max_val and const data attribute values
whereby fp16 values are stored as 2 bytes each, fp8 values are stored
in 1 byte each, etc.

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: Ia95d320fe8c546ce1d1ccc035d6e9bcaadcc9ca3