s390-bios: Map low core memory

Create a new header for basic architecture specific definitions and add a
mapping of low core memory. This mapping will be used by the real dasd boot
process.

Signed-off-by: Jason J. Herne <[email protected]>
Acked-by: Cornelia Huck <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-Id: <1554388475[email protected]>
Signed-off-by: Thomas Huth <[email protected]>