riscv: Fix use of undefined config option CONFIG_CONFIG_MMU

In Kconfig files, config options are written without the CONFIG_ prefix.

Fixes: 6bd33e1ece52 ("riscv: add nommu support")
Signed-off-by: Andreas Schwab <schwab@suse.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 759ffb0..d8efbaa 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -154,7 +154,7 @@
 	def_bool y
 
 config FIX_EARLYCON_MEM
-	def_bool CONFIG_MMU
+	def_bool MMU
 
 config PGTABLE_LEVELS
 	int