riscv: Rename cpu/qemu to cpu/generic

riscv: Rename cpu/qemu to cpu/generic

The QEMU CPU support under arch/riscv is pretty much generic
and works fine for SiFive Unleashed as well. In fact, there
will be quite a few RISC-V SOCs for which QEMU CPU support
will work fine.

This patch renames cpu/qemu to cpu/generic to indicate the
above fact. If there are SOC specific errata workarounds
required in cpu/generic then those can be done at runtime
in cpu/generic based on CPU vendor specific DT compatible
string.

Signed-off-by: Anup Patel <[email protected]>
Reviewed-by: Alexander Graf <[email protected]>
Reviewed-by: Lukas Auer <[email protected]>
Reviewed-by: Bin Meng <[email protected]>