armv8: Make COUNTER_FREQUENCY optional

Some platforms have the means to determine the counter frequency at
runtime, so give them an opportunity to do so.
Signed-off-by: NThierry Reding <treding@nvidia.com>
Acked-by: NStephen Warren <swarren@nvidia.com>
Tested-by: NStephen Warren <swarren@nvidia.com>
Signed-off-by: NTom Warren <twarren@nvidia.com>

arch/arm/cpu/armv8/start.S

@@ -54,8 +54,10 @@ reset:
 	orr	x0, x0, #0xf			/* SCR_EL3.NS|IRQ|FIQ|EA */
 	msr	scr_el3, x0
 	msr	cptr_el3, xzr			/* Enable FP/SIMD */
+#ifdef COUNTER_FREQUENCY
 	ldr	x0, =COUNTER_FREQUENCY
 	msr	cntfrq_el0, x0			/* Initialize CNTFRQ */
+#endif
 	b	0f
 2:	msr	vbar_el2, x0
 	mov	x0, #0x33ff