clocksource/drivers/arm_arch_timer: Fix vDSO clockmode when vDSO disabled

The arm_arch_timer requires VDSO_CLOCKMODE_ARCHTIMER to be defined to
compile correctly. On ARM the vDSO can be disabled and when this is the
case the compilation ends prematurely with an error:

 $ make ARCH=arm multi_v7_defconfig
 $ ./scripts/config -d VDSO
 $ make

drivers/clocksource/arm_arch_timer.c:73:44: error:
‘VDSO_CLOCKMODE_ARCHTIMER’ undeclared here (not in a function)
  static enum vdso_clock_mode vdso_default = VDSO_CLOCKMODE_ARCHTIMER;

Make the usage of VDSO_CLOCKMODE_ARCHTIMER depend on the VDSO enablement
and initialize the vdso clockmode variable with VDSO_CLOCKMODE_NONE
otherwise.

[ tglx: Match changelog and patch content. ]

Fixes: 5e3c6a312a ("ARM/arm64: vdso: Use common vdso clock mode storage")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200224151552.57274-1-vincenzo.frascino@arm.com

drivers/clocksource/arm_arch_timer.c

@@ -69,7 +69,11 @@ static enum arch_timer_ppi_nr arch_timer_uses_ppi = ARCH_TIMER_VIRT_PPI;
 static bool arch_timer_c3stop;
 static bool arch_timer_mem_use_virtual;
 static bool arch_counter_suspend_stop;
+#ifdef CONFIG_GENERIC_GETTIMEOFDAY
 static enum vdso_clock_mode vdso_default = VDSO_CLOCKMODE_ARCHTIMER;
+#else
+static enum vdso_clock_mode vdso_default = VDSO_CLOCKMODE_NONE;
+#endif /* CONFIG_GENERIC_GETTIMEOFDAY */
 
 static cpumask_t evtstrm_available = CPU_MASK_NONE;
 static bool evtstrm_enable = IS_ENABLED(CONFIG_ARM_ARCH_TIMER_EVTSTREAM);