time: Add T_First constant

The constant can be used to initialize variables of type HW.Time.T.

Change-Id: I65246d6776d464bfa8648075b48e7de61cd82902
Signed-off-by: Adrian-Ken Rueegsegger <ken@codelabs.ch>
Reviewed-on: https://review.coreboot.org/c/libhwbase/+/38983
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Nico Huber <nico.h@gmx.de>

common/hw-time.ads

@@ -18,6 +18,8 @@ is
 
    type T is private;
 
+   T_First : constant T;
+
    function Now return T
    with
       Volatile_Function,
@@ -71,6 +73,8 @@ private
 
    type T is new Word64;
 
+   T_First : constant T := T'First;
+
 end HW.Time;
 
 --  vim: set ts=8 sts=3 sw=3 et: