[POWERPC] Move of_device allocation into of_device.[ch]

Extract generic of_device allocation code from of_platform_device_create()
and move it into of_device.[ch], called of_device_alloc(). Also, there's now
of_device_free() which puts the device node.

This way, bus drivers that build on of_platform (like ibmebus will) can
build upon this code instead of reinventing the wheel.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>