riscv: Call spl_board_init_f() in the generic SPL board_init_f()

The generic SPL version of board_init_f() should give a call to
board specific codes to initialize board in the SPL phase.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Tested-by: Pragnesh Patel <pragnesh.patel@sifive.com>