drivers/rtc/rtc-at91sam9.c: use module_platform_driver() macro

This driver does seems to do only platform_driver_register in the init
function and platform_driver_unregister in the exit function,

so replace all this code including the module_init and module_exit with
module_platform_driver macro...

Signed-off-by: Devendra Naga <develkernel412222@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>