[arm] Use CNTVCT_EL0 as profiling timestamp

The raw cycle counter at PMCCNTR_EL0 works in qemu but seems to always
read as zero on physical hardware (tested on Juno r1 and Cavium
ThunderX), even after ensuring that PMCR_EL0.E and PMCNTENSET_EL0.C
are both enabled.

Use CNTVCT_EL0 instead; this seems to count at a lower resolution
(tens of CPU cycles), but is usable for profiling.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/arm64/include/bits/profile.h

@@ -21,9 +21,7 @@ profile_timestamp ( void ) {
 	uint64_t cycles;
 
 	/* Read cycle counter */
-	__asm__ __volatile__ ( "msr PMCR_EL0, %1\n\t"
+	__asm__ __volatile__ ( "mrs %0, CNTVCT_EL0\n\t" : "=r" ( cycles ) );
-			       "mrs %0, PMCCNTR_EL0\n\t"
-			       : "=r" ( cycles ) : "r" ( 1 ) );
 	return cycles;
 }