libata: pata_of_platform: OF-Platform PATA device driver

This driver nicely wraps around pata_platform library functions,
and provides OF platform bus bindings to the PATA devices.

Also add || PPC to the PATA_PLATFORM's "depends on" Kconfig entry,
needed for PA Semi Electra.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Olof Johansson <olof@lixom.net>