arm-autonomy/xen-devicetree: Lower log level for incorrect kernel size

Reduced the level at which messages relating to overriding the user provided
value for XEN_DEVICETREE_DOM0_SIZE when it is too small are logged at from
'warn' to 'note'

Issue-Id: SCM-2037
Signed-off-by: Nathan Dunne <Nathan.Dunne@arm.com>
Change-Id: I1436dac2c347d40c897291220fe30a95a57f2fa1
Signed-off-by: Jon Mason <jon.mason@arm.com>