arm64: Use arch_timer_get_rate when trapping CNTFRQ_EL0

In an ideal world, CNTFRQ_EL0 always contains the timer frequency
for the kernel to use. Sadly, we get quite a few broken systems
where the firmware authors cannot be bothered to program that
register on all CPUs, and rely on DT to provide that frequency.

So when trapping CNTFRQ_EL0, make sure to return the actual rate
(as known by the kernel), and not CNTFRQ_EL0.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/traps.c

@@ -523,7 +523,7 @@ static void cntfrq_read_handler(unsigned int esr, struct pt_regs *regs)
 {
 	int rt = (esr & ESR_ELx_SYS64_ISS_RT_MASK) >> ESR_ELx_SYS64_ISS_RT_SHIFT;
 
-	pt_regs_write_reg(regs, rt, read_sysreg(cntfrq_el0));
+	pt_regs_write_reg(regs, rt, arch_timer_get_rate());
 	regs->pc += 4;
 }