fwk_arch: Fix buffered log output behaviour

When the SCP is compiled as a subsystem `SCP_ENABLE_SUB_SYSTEM_MODE` is
enabled, logging is not printed correctly. In this mode, only one
character is printed instead of the complete log message. This defect
is fixed by replacing `fwk_log_unbuffer` with `fwk_log_flush`.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: If7ae17aad66e252db823b690c794ca6b5620e882