ARM: mach-shmobile: Move sh7372 AP4EVB external clk setup

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: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>