arm64: Add CNTPOFF_EL2 register definition

Add the definition for CNTPOFF_EL2 in the description file.

Reviewed-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230330174800.2677007-3-maz@kernel.org