[ARM] pxa: define zeus_power_off() only when CONFIG_PM enabled

Add some safety check for CONFIG_PM around zeus_power_off(). Without
it linking can fail like this:

arch/arm/mach-pxa/built-in.o: In function `zeus_power_off':
e800.c:(.text+0x2bc8): undefined reference to `pxa27x_cpu_suspend'
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Acked-by: Marc Zyngier <maz@misterjones.org>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>

arch/arm/mach-pxa/zeus.c

@@ -621,11 +621,15 @@ static struct pxa2xx_udc_mach_info zeus_udc_info = {
 	.udc_command = zeus_udc_command,
 };
 
+#ifdef CONFIG_PM
 static void zeus_power_off(void)
 {
 	local_irq_disable();
 	pxa27x_cpu_suspend(PWRMODE_DEEPSLEEP);
 }
+#else
+#define zeus_power_off   NULL
+#endif
 
 #ifdef CONFIG_APM_EMULATION
 static void zeus_get_power_status(struct apm_power_info *info)