SUPPORT.md: add Dom0less as Supported

SUPPORT.md: add Dom0less as Supported

Add Dom0less to SUPPORT.md to clarify its support status. The feature is
mature enough and small enough to make it security supported.

Clarify that dom0less DomUs memory is not scrubbed at boot when
bootscrub=on or bootscrub=off are passed as Xen command line parameters,
and no XSAs will be issued for that.

Also see XSA-372: 371347c5b64da and fd5dc41ceaed.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>