juno/reg_sensor: Change the type of the fake_register to uint64_t

Created by: raphaelgault

The value read when doing a get_value is 64-bit so we need to store a 64-bit value in the fake register.

Change-Id: I3cf451e2659436106cc93605928fde527045ac23 Signed-off-by: Raphael Gault raphael.gault@arm.com