include/cpu/amd/mtrr: rename functions to get top of memory regions

Rename amd_topmem and amd_topmem2 to get_top_of_mem_below_4gb and
get_top_of_mem_above_4g to make it clearer what those functions return.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: Ic6e98d94c731af74aea0ce276a9a7e4867e3986f
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74589
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martin.roth@amd.corp-partner.google.com>

src/soc/amd/stoneyridge/northbridge.c

@@ -276,7 +276,7 @@ struct device_operations stoneyridge_northbridge_operations = {
  */
 void amd_initcpuio(void)
 {
-	uintptr_t topmem = amd_topmem();
+	uintptr_t topmem = get_top_of_mem_below_4gb();
 	uintptr_t base, limit;
 
 	/* Enable legacy video routing: D18F1xF4 VGA Enable */