Make errata reporting mandatory for CPU files

Make errata reporting mandatory for CPU files

Previously the errata reporting was optional for CPU operation
files and this was achieved by making use of weak reference to
resolve to 0 if the symbol is not defined. This is error prone
when adding new CPU operation files and weak references are
problematic when fixing up dynamic relocations. Hence this patch
removes the weak reference and makes it mandatory for the CPU
operation files to define the errata reporting function.

Change-Id: I8af192e19b85b7cd8c7579e52f8f05a4294e5396
Signed-off-by: Soby Mathew <soby.mathew@arm.com>