powerpc, of_serial: Endianness issues setting up the serial ports

The speed and clock of the serial ports is retrieved from the device
tree in both the PowerPC legacy serial code and the Open Firmware serial
driver, therefore they need to handle the fact that the device tree is
always big endian, while the CPU may not be.

Also fix other device tree references in the legacy serial code.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>