config: ARM: Change RTC drivers to built-in
    
    Consistently do not build RTC drivers as modules. This fixes subtle
    differences between armv7hl default and lpae flavors and avoids any
    timing-related driver initialization issues.
    
    To make RTC_DRV_88PM80X built-in, make MFD_88PM800 built-in, too.
    To make RTC_DRV_PM8XXX built-in, make MFD_PM8921_CORE built-in, too.
    
    Suggested-by: Alexander Graf <agraf@suse.de>
    Signed-off-by: Andreas Färber <afaerber@suse.de>