ARM: dts: aspeed: Make G5 clocks fixed

We do not yet have a clk driver upstream. So that users can boot the
unmodified upstream kernel, add fixed-clock and clock-frequency
properties to all of the clocks.

The values are taken from the ast2500evb. This is the only upstream dts.
It also happens to match all of the systems I have seen so far.

Acked-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>