target/riscv: Disable the MMU correctly

target/riscv: Disable the MMU correctly

Previously if we didn't enable the MMU it would be enabled in the
realize() function anyway. Let's ensure that if we don't want the MMU we
disable it. We also don't need to enable the MMU as it will be enabled
in realize() by default.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>