timer: sandbox: work without device tree

A default invocation of sandbox U-Boot apparently uses no device tree,
which means that no timer is registers, which in turn means that the
sleep shell command hangs.

Fix the sandbox timer code to register a device when there's no DT, just
like e.g. the sandbox reset driver does. When there's no DT, the DM uclass
can't initialize clock_rate from DT, so set a default value in the
timer code instead.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>