AArch64: Use long for pointers in MTP classes

For storing pointers, long is used in MTP classes,
as native pointers can be 64-bit.

In addition, some minor changes have been done
to conform with standard JNI practice (e.g. use
of jint instead of int in JNI function prototypes)

Change-Id: I67805547251722e7b77611d47d0bb632a64d3e6d
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>
Signed-off-by: Marcus Oakland <marcus.oakland@arm.com>
Signed-off-by: Kévin PETIT <kevin.petit@arm.com>