muen-timer: Set correct clock event feature flags

Use CLOCK_EVT_FEAT_ONESHOT to set clock event feature flag. The
CLOCK_EVT_MODE_ONESHOT enum value has a different value than the flag
and must not be used in this context.
muen-timer: Initial clock event using timer page

Use Muen subject timer page to set next event. Only one-shot mode is
supported. Currently the address of the timer page is hard-coded to
0xe00000000 for now.
muen-timer: Warn when setting unsupported mode
muen-timer: Raise priority of muen-timer clock event driver

Make sure that Muen Timer is used as clock event device, if present.