ARM: mach-shmobile: add shmobile_earlytimer_init()

Add shmobile_earlytimer_init() that can be used to
enable the earlytimer probing from the SoC code.

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