BL31_CACHE_DISABLE flag was introduced as a work-around
for the older SoC revisions. Since it is not relevant in the
newest versions, toggle it to be disabled by default.
One can still specify it by adding 'BL31_CACHE_DISABLE=1'
string to the build command.

Change-Id: I11b52dade3ff7f8ee643b8078c6e447c45946570
Signed-off-by: Marcin Wojtas <mw@semihalf.com>