Resolve TZC400 build issue when DEBUG=1 and ENABLE_ASSERTIONS=0

Previously the definition of `_tzc_read_peripheral_id()` was wrapped
in ENABLE_ASSERTIONS build flag. This causes build issue for TZC400 driver
when DEBUG=1 and ENABLE_ASSERTIONS=0. This patch fixes the same by
moving the definitions outside the ENABLE_ASSERTIONS build flag.

Change-Id: Ic1cad69f02ce65ac34aefd39eaa96d5781043152
Signed-off-by: Soby Mathew <soby.mathew@arm.com>