uboot-imx

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>