lib/cbmem_top: Add a common cbmem_top implementation

This adds a common cbmem_top implementation to all coreboot target.

In romstage a static variable will be used to cache the result of
cbmem_top_romstage.

In ramstage if CONFIG_RAMSTAGE_CBMEM_TOP_ARG is set a global variable
needs to be populated by the stage entry with the value passed via the
calling arguments. if CONFIG_RAMSTAGE_CBMEM_TOP_ARG is not set the
same implementation as will be used as in romstage.

Change-Id: Ie767542ee25483acc9a56785ce20a885e9a63098
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36273
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/northbridge/intel/pineview/memmap.c

@@ -132,7 +132,7 @@ static uintptr_t northbridge_get_tseg_base(void)
  * 1 MiB alignment. As this may cause very greedy MTRR setup, push
  * CBMEM top downwards to 4 MiB boundary.
  */
-void *cbmem_top(void)
+void *cbmem_top_chipset(void)
 {
 	uintptr_t top_of_ram = ALIGN_DOWN(northbridge_get_tseg_base(), 4*MiB);
 	return (void *) top_of_ram;