rtc: fsl-ftm-alarm: Use module_platform_driver replace device_initcall

rtc: fsl-ftm-alarm: Use module_platform_driver replace device_initcall

The ftm_rtc_driver has been registered while module init, however there
is not unregister step for module exit, now use the macro
module_platform_driver replace device_initcall, which can register and
unregister platform driver automatically.

Signed-off-by: Zhang Jianhua <chris.zjh@huawei.com>
Link: https://lore.kernel.org/r/20220906143037.1455317-1-chris.zjh@huawei.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>