acpigen: Add acpigen_emit_eisaid.

Change-Id: Ib92142a133445018cd152dabe299792ba5f36548
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/5240
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>