driver: xen: Replace cpu_up/down with device_online/offline

The core device API performs extra housekeeping bits that are missing
from directly calling cpu_up/down.

See commit a6717c01 ("powerpc/rtas: use device model APIs and
serialization during LPM") for an example description of what might go
wrong.

This also prepares to make cpu_up/down a private interface for anything
but the cpu subsystem.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>