debian: Avoid flooding the console with systemd messages

The systemd log level defaults to debug when "debug" is
passed on the kernel command line, as is done in our setupe.
Setting the systemd log level to info reduces the verbosity.

Without this change there is a constant stream of:
systemd-journald[201]: Sent WATCHDOG=1 notification.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>