RISC-V: Only select essential drivers for SOC_VIRT config

The kconfig select causes build failues for SOC_VIRT config becaus
we are selecting lot of VIRTIO drivers without selecting all required
dependencies.

Better approach is to only select essential drivers from SOC_VIRT
config option and enable required VIRTIO drivers using defconfigs.

Fixes: 759bdc16

 ("RISC-V: Add kconfig option for QEMU virt machine")
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>