Rename FVP "mmap" array to avoid name confusion

Rename the array "mmap" in plat/fvp/aarch64/plat_common.c to
"fvp_mmap", to avoid confusion with the array of the same name
in lib/arch/aarch64/xlat_tables.c

Fixes ARM-software/tf-issues#114

Change-Id: I61478c0070aa52d5dcf5d85af2f353f56c060cfb