refactor(synquacer): move common source files

Prepare for introduction of BL2 support by moving
reusable files from BL31_SOURCES into PLAT_BL_COMMON_SOURCES

Cc: Sumit Garg <sumit.garg@linaro.org>
Cc: Masahisa Kojima <masahisa.kojima@linaro.org>
Cc: Manish V Badarkhe <manish.badarkhe@arm.com>
Cc: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I21137cdd40d027cfa77f1dec3598ee85d4873581
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>