arm: socfpga: Do not call board_init_r() from board_init_f()

Instead of calling board_init_r() directly from board_init_f(), just
return from board_init_f(). This will make the code continue executing
in crt0.S _main(), from which the board_init_r() is called. This patch
aligns the SoCFPGA SPL with the correct SPL design as well as reduces
the stack utilisation slightly.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>