timestamps: Improve collection for ENV_ROMSTAGE_OR_BEFORE

Keep track of the active timestamp table location using
a CAR_GLOBAL variable. Done this way, the entire table
can be located outside _car_relocatable_data and we only
switch the pointer to CBMEM and copy the data before
CAR gets torn down.

Fix comments about requirements of timestamp_init() usage.

Remove timestamp_cache from postcar and ramstage, as CBMEM
is available early on.

Change-Id: I87370f62db23318069b6fd56ba0d1171d619cb8a
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35032
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/timestamp.h

@@ -21,12 +21,10 @@
 
 #if CONFIG(COLLECT_TIMESTAMPS)
 /*
- * timestamp_init() needs to be called once for each of these cases:
- *    1. __PRE_RAM__ (bootblock, romstage, verstage, etc) and
- *    2. !__PRE_RAM__ (ramstage)
- * The latter is taken care of by the generic coreboot infrastructure so
- * it's up to the chipset/arch to call timestamp_init() in *one* of
- * the __PRE_RAM__ stages. If multiple calls are made timestamps will be lost.
+ * timestamp_init() needs to be called once in *one* of the ENV_ROMSTAGE_OR_BEFORE
+ * stages (bootblock, romstage, verstage, etc). It's up to the chipset/arch
+ * to make the call in the earliest stage, otherwise some timestamps will be lost.
+ * For x86 ENV_ROMSTAGE call must be made before CAR is torn down.
  */
 void timestamp_init(uint64_t base);
 /*