arm64: timer: Create timer_get_bootus for bootstage support

Implement timer_get_boot_us() based on available functions to support
bootstage command.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/cpu/armv8/generic_timer.c

@@ -61,3 +61,10 @@ unsigned long usec2ticks(unsigned long usec)
 
 	return ticks;
 }
+
+ulong timer_get_boot_us(void)
+{
+	u64 val = get_ticks() * 1000000;
+
+	return val / get_tbclk();
+}