cpuidle: riscv-sbi: Opt-out from genpd's common ->sync_state() support

The riscv-sbi-domain implements its own specific ->sync_state() callback.
Let's set the GENPD_FLAG_NO_SYNC_STATE to inform genpd about it.

Moreover, let's call of_genpd_sync_state() to make sure genpd tries to
power off unused PM domains.

Cc: Anup Patel <anup@brainfault.org>
Cc: linux-riscv@lists.infradead.org
Tested-by: Hiago De Franco <hiago.franco@toradex.com> # Colibri iMX8X
Tested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com> # TI AM62A,Xilinx ZynqMP ZCU106
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Rahul Pathak <rpathak@ventanamicro.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20250701114733.636510-12-ulf.hansson@linaro.org