armclang: Suppress warnings about unused sections

Not all defined scatter file sections will be used by each
example application, by design. So to avoid confusion about unused
sections in the build output, add L6314W to the list of warnings
to be suppressed.

Change-Id: I97b21c3e80cc8528a5207c5e3994a862b5548f9e
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>