hwmon: (xgene) Fix crash when alarm occurs before driver probe

The system crashes during probing xgene-hwmon driver when temperature
alarm interrupt occurs before.
It's because
 - xgene_hwmon_probe() requests mailbox channel which also enables
   the mailbox interrupt.
 - As temperature alarm interrupt is pending, ISR runs and crashes when
   accesses into invalid resourse as unmapped PCC shared memory.

This patch fixes this issue by saving this alarm message and scheduling a
bottom handler after xgene_hwmon_probe() finish.

Signed-off-by: Hoan Tran <hotran@apm.com>
Reported-by: Itaru Kitayama <itaru.kitayama@riken.jp>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>