rtc: brcmstb-waketimer: switch to rtc_register_device

Switch to devm_rtc_allocate_device/rtc_register_device.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>