ARM: mach-shmobile: r8a7779 and Marzen timer rework

Move the SoC specific timer code from Marzen board code
to r8a7779 setup code. This makes is possible to share
the SoC specific timer code across boards and it also
removes the need for a board specific timer structure.

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