cpuidle: riscv-sbi: Drop redundant sync_state support

The recent updates to the genpd core, can entirely manage the sync_state
support for the cpuidle-riscv-sbi-domain. More precisely, genpd prevents
our ->power_off() callback from being invoked, until all of our consumers
are ready for it.

Let's therefore drop the sync_state support for the
cpuidle-riscv-sbi-domain as it has become redundant.

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: Rahul Pathak <rpathak@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20250701114733.636510-25-ulf.hansson@linaro.org