riscv: Fix use of undefined config option CONFIG_CONFIG_MMU

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

Fixes: 6bd33e1e ("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>

arch/riscv/Kconfig

@@ -154,7 +154,7 @@ config GENERIC_HWEIGHT
 	def_bool y
 
 config FIX_EARLYCON_MEM
-	def_bool CONFIG_MMU
+	def_bool MMU
 
 config PGTABLE_LEVELS
 	int