[U-Boot] [U-Boot, 24/36] armv8: add timer_get_boot_us() for generic timer

[Philipp Tomsich]

> We need timer_get_boot_us() for boot stage if we use generic timer only.
> 
> Signed-off-by: Kever Yang <kever.yang at rock-chips.com>
> ---
> 
>  arch/arm/cpu/armv8/generic_timer.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 

Acked-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>