AGESA,binaryPI: Split romstage_main() to BSP and AP parts

BSP and AP have two distinct execution paths for romstage.

Change-Id: Id013b165f1345509fe6b74cef2bf8c3b420f84a4
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37326
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/cpu/amd/car.h

@@ -3,6 +3,6 @@
 
 #include <arch/cpu.h>
 
-void *asmlinkage romstage_main(unsigned long bist);
+asmlinkage void romstage_main(unsigned long bist);
 
 #endif