RISC-V: Only default to spinwait on SBI-0.1 and M-mode

RISC-V: Only default to spinwait on SBI-0.1 and M-mode

The spinwait boot method has been superseded by the SBI HSM extension
for some time now, but it still enabled by default.  This causes some
issues on large hart count systems, which will hang if a physical hart
exists that is larger than NR_CPUS.

Users on modern SBI implementation don't need spinwait, and while it's
probably possible to deal with some of the spinwait issues let's just
restrict the default to systems that are likely to actually use it.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20220421170354.10555-1-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>