s390/idle: reduce size of s390_idle_data structure

Avoid padding in the middle and the end of the data structure by
moving nohz_delay member to the beginning.
Saves eight byte per cpu.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>