KVM: arm64: selftests: Augment existing timer test to handle variable offset

Allow a user to specify the global offset on the command-line.

Reviewed-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230330174800.2677007-21-maz@kernel.org