sam460ex: Fix sam460ex device tree when booting the Linux kernel

sam460ex: Fix sam460ex device tree when booting the Linux kernel

sam460ex (or at least this emulation) does not support the "ibm,cpm" power
management. As a result, Linux crashes when trying to access it. Remove
its device tree node. Also, if/when we boot the Linux kernel directly,
serial port clock frequencies in the device tree file will be unset, and
serial port initialization will fail. Add valid frequency values to
the serial ports to be able to use it. Also set valid values for the other
clock nodes otherwise set by u-boot.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>