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>