fwk: rename `FWK_LOG_LEVEL_TRACE` to `FWK_LOG_LEVEL_DEBUG`

This patch renames `TRACE` to `DEBUG` because in further patches
a new functionallity for tracing will be introduced and having
the same name for both could cause confusion.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: I42517984982fbcb3e9f325f89a9ca7761852831b