riscv: use imply instead of select for SPL_SEPARATE_BSS

Use imply instead of select, then it can still be disabled by
board-specific defconfig, or be set to n manually.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>