clk: aspeed: Register core clocks

This registers the core clocks; those which are required to calculate
the rate of the timer peripheral so the system can load a clocksource
driver.

Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>