Fix int4 and int48 deserialization

Previously we were using isSigned=false which would raise an error on
negative values. Also, int4 was being cast to uint8_t, which meant that
then it would get implicily zero-extended to uint64_t and representing
the wrong number when interpreted as a signed value.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I9770d896efe55b5f8fe66a12754d6ed69a3165db