utils/trace: fix parsing starting from trace.txt
utils/trace: fix parsing starting from trace.txt

When a trace TXT file is specified to the Trace class, we fails the
parsing because the functions profiler code tries to load function stats
from a non existing file.

Fix this issue by not assuming that the specified trace.txt file is
a JSON containing the functions statistics.

Signed-off-by: Patrick Bellasi <431c86f368affbc3148c4669fbd465131f7e82cb@arm.com>

mdigiorgio/lisa,bjackman/lisa,mdigiorgio/lisa,bjackman/lisa,credp/lisa,credp/lisa,credp/lisa,arnoldlu/lisa,arnoldlu/lisa,ARM-software/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa