riscv: Make stack size shift configurable

Add prompt for STACK_SIZE_SHIFT to make it configurable.
The default value remains 14 as usual.

Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index c5859c5..d0476f9 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -453,7 +453,7 @@
 	  memory is configured was also changed.
 
 config STACK_SIZE_SHIFT
-	int
+	int "Stack size shift"
 	default 14
 
 config OF_BOARD_FIXUP