ubifs: Remove ifdefs around CONFIG_UBIFS_ATIME_SUPPORT

ifdefs reduce readability and compile coverage. This removes the ifdefs
around CONFIG_UBIFS_ATIME_SUPPORT by replacing them with IS_ENABLED()
where applicable. The fs layer would fall back to generic_update_time()
when .update_time doesn't exist. We do this fallback explicitly now.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>