riscv: Add SiFive drivers to rv32_defconfig

This adds SiFive drivers to rv32_defconfig, to keep in sync with the
64-bit config. This is useful when testing 32-bit kernel with QEMU
'sifive_u' 32-bit machine.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>