rockchip: timer: add device-model timer driver for RK3368 (and similar)

This adds a device-model driver for the timer block in the RK3368 (and
similar devices that share the same timer block, such as the RK3288) for
the down-counting (i.e. non-secure) timers.

This allows us to configure U-Boot for the RK3368 in such a way that
we can run with the secure timer inaccessible or uninitialised (note
that the ARMv8 generic timer does not count, if the secure timer is
not enabled).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>