arm-bsp/conf/fvp-rd-kronos: Handle FVP shutdown request

Currently, there is no way to shutdown the FVP
through software.

Overcome this by using the FVP shutdown_tag parameter.
When the shutdown request is complete, The RSS prints
a certain message in its terminal which gets detected
by the FVP causing the FVP to terminate.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>