arm64: arch_timer: Make workaround methods optional

Not all errata need to workaround all access types. Allow them to
be optional.
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/include/asm/arch_timer.h

@@ -60,8 +60,9 @@ extern const struct arch_timer_erratum_workaround *timer_unstable_counter_workar
 #define arch_timer_reg_read_stable(reg) 		\
 ({							\
 	u64 _val;					\
-	if (needs_unstable_timer_counter_workaround())		\
-		_val = timer_unstable_counter_workaround->read_##reg();\
+	if (needs_unstable_timer_counter_workaround() &&		\
+	    timer_unstable_counter_workaround->read_##reg)		\
+		_val = timer_unstable_counter_workaround->read_##reg();	\
 	else						\
 		_val = read_sysreg(reg);		\
 	_val;						\