Change to use uint32_t in Arm profiler for ticks

Currently the profiler uses signed 32-bit integers to hold the start and
end tick values. This is not appropriate because the tick values will
never be negative and the current ticks are returned as an unsigned
32-bit integer value.

To address this, the start and end ticks will now be stored as unsigned
32-bit integers instead.

Change-Id: I6358f3fde7c034c5e40409a9d3f7a576ff4897ce
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>