riscv: Use register_platform_power_off()

Utilize new register_platform_power_off() API that allows to register
multiple power-off handlers and deprecates global pm_power_off callback.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>

arch/riscv/kernel/sbi.c

@@ -207,7 +207,7 @@ static int __sbi_rfence_v01(int fid, const struct cpumask *cpu_mask,
 
 static void sbi_set_power_off(void)
 {
-	pm_power_off = sbi_shutdown;
+	register_platform_power_off(sbi_shutdown);
 }
 #else
 static void __sbi_set_timer_v01(uint64_t stime_value)