arm/trusted-firmware-a: disable branch protection on SGI575

oe-core master's gcc is built with branch protection on by default. However,
this results in slightly larger binaries which mean the SGI575 build is now
too large.

Until this is investigated properly, we can just disable branch protection
for the SGI575 build.

Change-Id: I9a8b0a8e569b944c274199ce306a3864c94c8c5c
Signed-off-by: Ross Burton <ross.burton@arm.com>