stm32mp1: split code between common and private parts

Some parts of code could be shared with platform derivatives,
or new platforms.
A new folder plat/st/common is created to put common parts.

stm32mp_common.h is a common API aggregate.

Remove some casts where applicable.
Fix some types where applicable.
Remove also some platform includes that are already in stm32mp1_def.h.

Change-Id: I46d763c8d9e15732d1ee7383207fd58206d7f583
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
diff --git a/plat/st/stm32mp1/stm32mp1_def.h b/plat/st/stm32mp1/stm32mp1_def.h
index 8cd5aeb..d12a93f 100644
--- a/plat/st/stm32mp1/stm32mp1_def.h
+++ b/plat/st/stm32mp1/stm32mp1_def.h
@@ -13,7 +13,8 @@
 
 #ifndef __ASSEMBLY__
 #include <boot_api.h>
-#include <stm32mp1_dt.h>
+#include <stm32mp_common.h>
+#include <stm32mp_dt.h>
 #include <stm32mp1_private.h>
 #endif