drivers/timer: MediaTek audio DSP timer device

These devices have a somewhat odd hybrid design, with a free-running
64 bit up counter but no comparator.  Instead interrupts are triggered
by (one of an array of) 32 bit down counters with reset (a-la SysTick,
but without the 24 bit precision issues).  The combination actually
results in a fairly simple driver as we can skip the comparator
rounding math.

Signed-off-by: Andy Ross <andyross@google.com>