riscv: remove CONFIG_RISCV_ISA_A

This option is always enabled, and not supporting the A extensions would
create a complete ABI trainwreck, so there is no point in even slightly
encouraging such an idea by keeping this unselectable code around.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>