riscv: defconfig: align RV64 defconfig to the output of "make savedefconfig"

Align the RV64 defconfig to the output of "make savedefconfig" to
avoid unnecessary deltas for future defconfig patches.  This patch
should have no runtime functional impact.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>