lib: memtest: Add MEMTEST_ENABLE_DEFAULT option

As of now memtest remains disabled until we specify the patterns
through the kernel command line. Some platforms have two
different configurations files (one for debug and another for
product) which can use the configuration option to enable the
memtest by default (in the debug configuration file).

CRs-Fixed: 1007344
Change-Id: I0bf7b33c3584f3d6cf5ef58dfe72be46212041da
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>