config: arch: riscv: add initial 64-bit support

Add initial rv64 support.  Originally based on suggestions from
Franz Flasch <franz.flasch@gmx.at>.

Cc: Franz Flasch <franz.flasch@gmx.at>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>

config/arch/riscv.in

@@ -4,9 +4,12 @@
 ## depends on EXPERIMENTAL
 ##
 ## select ARCH_SUPPORTS_32
+## select ARCH_SUPPORTS_64
 ## select ARCH_DEFAULT_32
+## select ARCH_SUPPORTS_BOTH_MMU
 ## select ARCH_SUPPORTS_WITH_ABI
 ## select ARCH_SUPPORTS_WITH_ARCH
+## select ARCH_SUPPORTS_WITH_TUNE
 ## select GCC_REQUIRE_7_or_later
 
 ## help The RISC-V architecture, as defined by: