clocksource: arch_timer: Only use the virtual counter (CNTVCT) on arm64

Commit 0b46b8a7 (clocksource: arch_timer: Fix code to use physical
timers when requested) introduces the use of physical counters in the
ARM architected timer driver. However, he arm64 kernel uses CNTVCT in
VDSO. When booting in EL2, the kernel switches to the physical timers to
make things easier for KVM but it continues to use the virtual counter
both in user and kernel. While in such scenario CNTVCT == CNTPCT (since
CNTVOFF is initialised by the kernel to 0), we want to spot firmware
bugs corrupting CNTVOFF early (which would affect CNTVCT).

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>