riscv: alternative only works on !XIP_KERNEL

The alternative mechanism needs runtime code patching, it can't work
on XIP_KERNEL. And the errata workarounds are implemented via the
alternative mechanism. So add !XIP_KERNEL dependency for alternative
and erratas.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Fixes: 44c92257

 ("RISC-V: enable XIP")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>