iris: Retry memory write in case of failure

An unknown error happens spuriously when multiple files are loaded into
the FVP in CI. The iris client only replies with the error `None` which
does not indicate the root cause of the error.
However experiments show that if the write is attempted again after some
delay it is likely to succeed.

This patch adds a mechanism to retry failed writes with the Iris flasher
Retry will stop after 5 failed attempts.

Signed-off-by: Vincent Coubard <vincent.coubard@arm.com>