booke_ppc: limit booke timer to max when timeout overflow

Limit watchdog and fit timer to maximum timeout value which
qemu timer can support (INT64_MAX). This maximum timeout will be
hundreds of years, so limiting to max timeout is pretty safe.

Signed-off-by: Bharat Bhushan <bharat.bhushan@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
