platform: Add a weak API to return the max peripherals supported by SPMI

Add a weak API to return the max SPMI peripherals supported. By default
the max number of peripherals supported on the platform is 128.

Change-Id: I17f14c1783378a6aff49dfa32d8265990029391f
diff --git a/platform/init.c b/platform/init.c
index 13612b7..9919b22 100644
--- a/platform/init.c
+++ b/platform/init.c
@@ -150,3 +150,7 @@
 {
 	return 0;
 }
+__WEAK uint32_t platform_get_max_periph()
+{
+	return 128;
+}