arm64: zero cntvoff_el2

arm64: zero cntvoff_el2

Currently cntvoff_el2 is initialised with an arbitrary bag of bits
derived from the initial value of cnthctl_el2 on the current CPU. This is
somewhat odd and problematic as some of these bits are UNKNOWN at reset
and may differ across CPUs (which may cause an OS at EL1 to observe time
going backwards across CPUs).

This patch instead initialises cntvoff_el2 with xzr, giving the register
a consistent value of zero on all CPUs.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: David Feng <fenghua@phytium.com.cn>
Cc: Tom Rini <trini@ti.com>
Acked-by: David.Feng <fenghua@phytium.com.cn>