riscv: sifive/fu540: spl: Drop our own version of board_init_f()

Use the generic board_init_f() provided by the RISC-V library codes.

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>