power: Add a clock_period variable to power expressions

power: Add a clock_period variable to power expressions

Currently 'Clock period in ticks' stat is not accessible in power equations .
This patch adds a variable 'clock_period' to be referenced to get the Clock period in
ticks

Signed-off-by: Sherif Elhabbal <elhabbalsherif@gmail.com>
Change-Id: Icac6a2e2003ed75d1680180e53343b0203139d72
Reviewed-on: https://gem5-review.googlesource.com/12664
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>