sandbox: physmem: Use mapping to support sandbox

Replace the raw cast with a map_sysmem() call so this code works with
sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/physmem.c b/lib/physmem.c
index f21ac24..84b191d 100644
--- a/lib/physmem.c
+++ b/lib/physmem.c
@@ -9,14 +9,16 @@
  */
 
 #include <common.h>
+#include <mapmem.h>
 #include <physmem.h>
 #include <linux/compiler.h>
 
 phys_addr_t __weak arch_phys_memset(phys_addr_t s, int c, phys_size_t n)
 {
-	void *s_ptr = (void *)(uintptr_t)s;
+	void *s_ptr = map_sysmem(s, n);
 
 	assert(((phys_addr_t)(uintptr_t)s) == s);
 	assert(((phys_addr_t)(uintptr_t)(s + n)) == s + n);
+
 	return (phys_addr_t)(uintptr_t)memset(s_ptr, c, n);
 }