systemc: Add some error checks to sc_clock.

The Accellera version reports an error if the period, high or low
portions of the clock are zero.

Change-Id: I2cbb7c3a1b6abe4d969ef875ebf8754d2d1c489a
Reviewed-on: https://gem5-review.googlesource.com/c/13303
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
1 file changed