arm64: sunxi: Build the sun4i timer driver

While the ARM architectural timer is generatlly the best timer to use,
a non-c3stop timer is needed for cpuidle.

Build the "sun4i" timer driver so it can be used for this purpose.
It is present on all 64-bit sunxi SoCs.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210322044707.19479-6-samuel@sholland.org