Change PMU event counter values to use 64-bit

The PMU event counter value is an accumulation of 32-bit values during
the inference and to ensure the total value fits in the rpmsg message
and UAPI, the variable holding the value has been changed to 64-bit.

The driver library, Python wrapper and inference runner have been
changed accordingly to support the 64-bit values.

Change-Id: I09a8e45eb75800c8a787f83abff5a3693148cc15
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>