dev-arm: VExpress_GEM5_Base, add refclock 32KHz

dev-arm: VExpress_GEM5_Base, add refclock 32KHz

This patch adds the reference 32KHz clock to VExpress_GEM5_Base derived
platforms. This is in preparation for supporting the SP805 Watchdog.
I/O voltage domain and platform clock domain coupling is transferred
to the __init__ method for correctness.

Change-Id: Ic743fd986793f1e43b75fa60260c9b43b2737763
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24204
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>