riscv: don't allow selecting SBI based drivers for M-mode

When running in M-mode we can't use SBI based drivers.  Add a new
CONFIG_RISCV_SBI that drivers that do SBI calls can depend on
instead.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>