riscv: Make A ISA extension selectable

Make the Atomic (A) ISA extension selectable. Thus CPUs such as
Ibex without the A extension can be supported.

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 fa3b016..c5859c5 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -308,7 +308,10 @@
 endmenu
 
 config RISCV_ISA_A
-	def_bool y
+	bool "Standard extension for Atomic Instructions"
+	default y
+	help
+	  Adds "A" to the ISA string passed to the compiler.
 
 config DMA_ADDR_T_64BIT
 	bool