Move the board specific external clock setting for sh7372 AP4EVB
from machine_desc->sys_timer->init() to machine_desc->init_machine().

This is ground work for shmobile timer code consolidation.
Signed-off-by: NMagnus Damm <damm@opensource.se>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>