altera_avalon_timer: new timer driver for Nios II

Tickless idle is not yet supported. We program the timer period
to the desired system clock tick rate (sys_clock_hw_cycles_per_tick).

This was renamed to the same name used in the Altera Embedded IP
Peripherals Guide; used by other CPUs than Nios II.

Change-Id: Ic4fca8c16b923295b77b63f98f45cd3483c5f560
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>