clk: aspeed: Add platform driver and register PLLs

This registers a platform driver to set up all of the non-core clocks.

The clocks that have configurable rates are now registered.

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>