rtc: rtc-pl031: Set wakeup flag prior to registering rtcdev

In some recent testing, I noticed the CLOCK_REALTIME_ALARM clockid
wasn't functioning on my vexpress qemu environment. Looking into it
I noticed the pl031 rtc driver doesn't set the wakeup flag on the
device until after registering the device with the RTC subsystem.

This causes the alarmtimer subsystem to not see the pl031 driver
as a valid backing device, and that resuls in alarm clockids
getting ENOTSUPP errors.

Thus be sure to set the wakeup flag on the device prior to
registering the rtcdev so the pl031 rtc driver can be used as
the backing alarmtimer device.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: "Jon Medhurst (Tixy)" <tixy@linaro.org>
Cc: linux-arm-kernel@lists.infradead.org
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>