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>

arch/Kconfig

@@ -111,7 +111,7 @@ config RISCV
 	select SUPPORT_OF_CONTROL
 	select OF_CONTROL
 	select DM
-	select SPL_SEPARATE_BSS if SPL
+	imply SPL_SEPARATE_BSS if SPL
 	imply DM_SERIAL
 	imply DM_ETH
 	imply DM_EVENT