at91: simplify spl board_init_f function

crt0.S do both memset the bss section and call board_init_r for us, so
remove them from board_init_f().

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>