ARM: 8947/1: Fix __arch_get_hw_counter() access to CNTVCT

ARM: 8947/1: Fix __arch_get_hw_counter() access to CNTVCT

__arch_get_hw_counter() should check clock_mode to see if it can access
CNTVCT. With the conversion to unified vDSO this check has been left out.

This causes on imx v6 and v7 (imx_v6_v7_defconfig) and other platforms to
hang at boot during the execution of the init process as per below:

[   19.976852] Run /sbin/init as init process
[   20.044931] Kernel panic - not syncing: Attempted to kill init!
exitcode=0x00000004

Fix the problem verifying that clock_mode is set coherently before
accessing CNTVCT.

Investigated-by: Arnd Bergmann <[email protected]>
Reported-by: Guenter Roeck <[email protected]>
Tested-by: Guenter Roeck <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Vincenzo Frascino <[email protected]>
Signed-off-by: Russell King <[email protected]>