base: Avoid dividing by zero in Time::[gs]etTick.

base: Avoid dividing by zero in Time::[gs]etTick.

Those functions used the value SimClock::Int::ns which, if the time
resolution is larger than 1ns, can/will be zero. That will make
getTick always return zero, and setTick divide by zero. This change
modifies those functions so that the math they do avoids using any
integer Ticks per time unit value except for Frequency. It seems
unlikely that the Ticks will increment at less than 1Hz.

Change-Id: I5cc9db14699c00dcbff48e4593b98522b13b4ccd
Reviewed-on: https://gem5-review.googlesource.com/12573
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>