soc: xilinx: Guard code which is called only for debugfs

soc: xilinx: Guard code which is called only for debugfs

Some functions are called only when debugfs for PM is enabled.
This patch removes compilation warnings:
drivers/soc/xilinx/zynqmp/pm.c:251:12: warning: ‘zynqmp_pm_self_suspend’
defined but not used [-Wunused-function]
 static int zynqmp_pm_self_suspend(const u32 node,
            ^
drivers/soc/xilinx/zynqmp/pm.c:300:12: warning:
‘zynqmp_pm_abort_suspend’ defined but not used [-Wunused-function]
 static int zynqmp_pm_abort_suspend(const enum zynqmp_pm_abort_reason
reason)
            ^
drivers/soc/xilinx/zynqmp/pm.c:494:12: warning:
‘zynqmp_pm_register_notifier’ defined but not used [-Wunused-function]
 static int zynqmp_pm_register_notifier(const u32 node, const u32 event,

Signed-off-by: Michal Simek <michal.simek@xilinx.com>