AMD K8 fam10-15: Tidy up CAR stack switch

Return to empty stack before making the switch.

Change-Id: I6d6f633933fac5bc08d9542c371715f737fb42cf
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/20574
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>

src/include/cpu/amd/car.h

@@ -4,10 +4,8 @@
 #include <arch/cpu.h>
 
 void cache_as_ram_main(unsigned long bist, unsigned long cpu_init_detectedx);
-void post_cache_as_ram(void);
-
-void cache_as_ram_switch_stack(void *stacktop);
-void cache_as_ram_new_stack(void);
+asmlinkage void * post_cache_as_ram(void);
+asmlinkage void cache_as_ram_new_stack(void);
 
 void disable_cache_as_ram(void);