kernel: calibrate the TWD timer even when booting with only one CPU, it is used for the watchdog timer base

Fixes booting on single-core CNS3xxx devices

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@36308 3c298f89-4303-0410-b956-a3cf2f4a3e73