clocksource/drivers/moxart: Add Aspeed support

The Aspeed SoC has timer IP with a very similar register layout to the
moxart timer. This patch adds support for the fourth and fifth gen
aspeed SoCs, and has been tested on the ast2400 and ast2500.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>