dev-arm: add FixedClock SimObject

dev-arm: add FixedClock SimObject

This patch adds a simple fixed-rate clock implementation based on
SrcClockDomain. This provides RealView-derived platform users with
a convenient way for auto-generating their platform clocks in the DTB.

Change-Id: Ifade0cc8ed1b9e3423745698442cac5d8b99ab63
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24223
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>