clk: Add clock driver for ASPEED BMC SoCs

This adds the stub of a driver for the ASPEED SoCs. The clocks are
defined and the static registration is set up.

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>