soc/intel/skylake: remove top_of_32bit_ram() declaration

It should never be globally exposed. Remove the global symbol
and make it static.

Change-Id: I3b85f3bbf6a73d480cdefdcdec26e137e3a3f75f
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/20584
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Subrata Banik <subrata.banik@intel.com>

src/soc/intel/skylake/memmap.c

@@ -137,7 +137,7 @@ int smm_subregion(int sub, void **start, size_t *size)
  * other words, the memory map is in a fixed order no matter what.
  */
 
-u32 top_of_32bit_ram(void)
+static u32 top_of_32bit_ram(void)
 {
 	msr_t prmrr_base;
 	u32 top_of_ram;