ARMv7: Factor out reusable timer_wait from sunxi/psci_sun7i.S

timer_wait is moved from sunxi/psci_sun7i.S, and it can be converted
completely into a reusable armv7 generic timer. LS1021A will use it
as well.

Signed-off-by: Wang Dongsheng <dongsheng.wang@freescale.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: York Sun <yorksun@freescale.com>