bootstage: Use rec_count as the array index

At present bootstage has a large array with all possible bootstage IDs
recorded. It adds times to the array element indexed by the ID. This is
inefficient because many IDs are not used during boot. We can save space
by only recording those IDs which actually have timestamps.

Update the array to use a record count, which increments with each
addition of a new timestamp. This takes longer to record a time, since it
may involve an array search. Such a search may be particularly expensive
before relocation when the CPU is running slowly or the cache is off. But
at that stage there should be very few records.

Signed-off-by: Simon Glass <sjg@chromium.org>