armv8: define usec2ticks function

usec2ticks() function has been defined for ARMv8 which will
be used by SEC Driver.

Signed-off-by: Aneesh Bansal <aneesh.bansal@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>

arch/arm/cpu/armv8/generic_timer.c

@@ -40,3 +40,14 @@ unsigned long timer_read_counter(void)
 #endif
 	return cntpct;
 }
+
+unsigned long usec2ticks(unsigned long usec)
+{
+	ulong ticks;
+	if (usec < 1000)
+		ticks = ((usec * (get_tbclk()/1000)) + 500) / 1000;
+	else
+		ticks = ((usec / 10) * (get_tbclk() / 100000));
+
+	return ticks;
+}