board: sifive: Fix a potential build warning in board_fdt_blob_setup()

board: sifive: Fix a potential build warning in board_fdt_blob_setup()

Commit 47d73ba4f4a4 ("board: sifive: overwrite board_fdt_blob_setup in u-boot proper")
added a board-specific implementation of board_fdt_blob_setup() which
takes a pointer as the return value, but it does not return anything
if CONFIG_OF_SEPARATE is not enabled. This will cause a build warning
seen when testing booting S-mode U-Boot directly from QEMU, per the
instructions in [1]:

  board/sifive/unleashed/unleashed.c: In function ‘board_fdt_blob_setup’:
  board/sifive/unleashed/unleashed.c:125:1: warning: control reaches end of non-void function [-Wreturn-type]

Return &_end as the default case.

[1] https://qemu.readthedocs.io/en/latest/system/riscv/sifive_u.html#running-u-boot

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Rick Chen <rick@andestech.com>