arm: socfpga: spl: Add s_init stub

Add a stub s_init function in the board file. The reason why the stub function
is needed is that most of the work is now being done in board_init_f(), there
is no need for the SPL to do anything s_init(). However, since lowlevel_init()
is still branching to s_init(), we need stub function for now, until
lowlevel_init() morphs into s_init().

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>

board/altera/socfpga/socfpga.c

@@ -18,6 +18,8 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
+void s_init(void) {}
+
 /*
  * Miscellaneous platform dependent initialisations
  */