Program loading: Handoff cbmem_top via calling arguments

There are a lot of different implementations to pass information from
romstage to ramstage. These could all be unified by passing this
information via cbmem. Often however these methods exist for that very
purpose. This solves this by passing cbmem_top via the programs
arguments.

Change-Id: Id2031f7bb81ce65fc318313c270eb1fbae3b2114
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36272
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/postcar_loader.c

@@ -225,5 +225,7 @@ void run_postcar_phase(struct postcar_frame *pcf)
 	/* As postcar exist, it's end of romstage here */
 	timestamp_add_now(TS_END_ROMSTAGE);
 
+	prog_set_arg(&prog, cbmem_top());
+
 	prog_run(&prog);
 }