riscv: sbi: Prefer SRST shutdown over legacy

riscv: sbi: Prefer SRST shutdown over legacy

Currently, the sbi_init() always attempts to register the legacy shutdown
function as the sys-off handler which is fine when RISCV_SBI_V01 is not
enabled. However, if RISCV_SBI_V01 is enabled in the kernel and the SBI
v0.1 is not supported by the underlying SBI implementation then the
legacy shutdown fails. Fix this by not registering the legacy shutdown
when SRST shutdown is available.

Fixes: 70ddf86d76c1 ("riscv: sbi: Switch to new sys-off handler API")
Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://patch.msgid.link/20251114065808.304430-1-mchitale@ventanamicro.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>