ARM: dts: aspeed: Add proper clock references

This device tree will break existing kernels that do not have the clk
patches applied (no clocksource, as we don't know the speed of the APB
clock.  You can boot if you pass a lpj value on the command line, but
won't have a uart).

Older device trees running with the newer kernel will function as well
as pre-4.16 kernels. That is, that some IP blocks (i2c, pwm/tach, adc)
will not work as the kernel lacks reset controller and clock enabling.

This is being changed as existing device trees use fixed-clocks in order
to boot without a clk driver. The newly added clk driver provides proper
clock support, including gating, so we move the device trees over to
properly request clocks.

The SCU compatible string is updated as the g4-scu string made it into
the tree before we decided on aspeed,astX000-<ip> as the format for the
strings. The old string will be removed from the bindings in a future
patch.

Signed-off-by: Joel Stanley <joel@jms.id.au>