spl_atmel.c: Switch s_init to board_init_f

spl_atmel.c: Switch s_init to board_init_f

To facilitate changing lowlevel_init to become s_init, move the current
contents of s_init into board_init_f and add the rest of what
board_init_f does here.
In order to compile clean without CONFIG_SKIP_LOWLEVEL_INIT set, leave an
empty stub of s_init(). It can be removed when lowlevel_init becomes s_init.

Cc: Bo Shen <[email protected]>
Cc: Andreas Bießmann <[email protected]>
Tested-by: Matt Porter <[email protected]> on sama5d3_xplained
Signed-off-by: Tom Rini <[email protected]>
[rebased on current master, leave s_init() as empty stub]
Signed-off-by: Andreas Bießmann <[email protected]>