openrisc: remove muldiv64()

openrisc: remove muldiv64()

Originally, timers were ticks based, and it made sense to
add ticks to current time to know when to trigger an alarm.

But since commit:

7447545 change all other clock references to use nanosecond resolution accessors

All timers use nanoseconds and we need to convert ticks to nanoseconds, by
doing something like:

    y = muldiv64(x, get_ticks_per_sec(), TIMER_FREQ)

where x is the number of device ticks and y the number of system ticks.

y is used as nanoseconds in timer functions,
it works because 1 tick is 1 nanosecond.
(get_ticks_per_sec() is 10^9)

But as openrisc timer frequency is 20 MHz, we can also do:

    y = x * 50; /* 20 MHz period is 50 ns */

Signed-off-by: Laurent Vivier <lvivier@redhat.com>