platform: Remove spike directory

The OpenSBI generic platform works perfectly fine on the QEMU spike
machine and Spike emulator so let's remove dedicated Spike platform
from OpenSBI.

All Spike platform related documentation in OpenSBI will now suggest
using OpenSBI generic platform.

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