arch/config: Make RISC-V 64-bits MMU optional

Linux supports No-MMU RISC-V 64-bits since kernel version 5.8. Make
MMU optional to enable building for RISC-V 64-bits boards that do not
have one. MMU use of RISC-V 32-bits builds remains mandatory for now.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

arch/Config.in

@@ -209,7 +209,6 @@ config BR2_powerpc64le
 
 config BR2_riscv
 	bool "RISCV"
-	select BR2_ARCH_HAS_MMU_MANDATORY
 	select BR2_ARCH_NEEDS_GCC_AT_LEAST_7
 	help
 	  RISC-V is an open, free Instruction Set Architecture created

arch/Config.in.riscv

@@ -71,10 +71,12 @@ choice
 
 config BR2_RISCV_32
 	bool "32-bit"
+	select BR2_ARCH_HAS_MMU_MANDATORY
 
 config BR2_RISCV_64
 	bool "64-bit"
 	select BR2_ARCH_IS_64
+	select BR2_ARCH_HAS_MMU_OPTIONAL
 
 endchoice