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/soc/sifive/fu540/cbmem.c

@@ -19,7 +19,7 @@
 #include <stdlib.h>
 #include <symbols.h>
 
-void *cbmem_top(void)
+void *cbmem_top_chipset(void)
 {
 	return (void *)min((uintptr_t)_dram + sdram_size_mb() * MiB,
 			   FU540_MAXDRAM);