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>

arch/arm/cpu/armv8/transition.S

@@ -43,7 +43,7 @@ ENTRY(armv8_switch_to_el1)
 	mrs	x0, cnthctl_el2
 	orr	x0, x0, #0x3		/* Enable EL1 access to timers */
 	msr	cnthctl_el2, x0
-	msr	cntvoff_el2, x0
+	msr	cntvoff_el2, xzr
 	mrs	x0, cntkctl_el1
 	orr	x0, x0, #0x3		/* Enable EL0 access to timers */
 	msr	cntkctl_el1, x0