ARM: mach-shmobile: sh73a0, AG5EVM and Kota2 timer rework

Move the SoC specific timer code from AG5EVM and Kota2
to sh73a0 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>