haven: Add support for the MEM_ACCEPT call

The MEM_ACCEPT call is required for VMs to map the
memory represented by a handle into their RM managed
page-tables, so add support for it.

Change-Id: I669ebd8298af405ca586a5721b26c4e19ffe39aa
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>