hw/timer/grlib_gptimer: Avoid integer overflows

The GPTIMER uses 32-bit registers.  Use a 64-bit operation to get the
ptimer count, otherwise we end up with a count of 0 for GPTIMER counter
values of 0xffffffff.

Use the GPTIMER counter value for tracing to avoid an overflow of the
32-bit value passed to trace_grlib_gptimer_enable().

Reviewed-by: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
