[POWERPC] Delete boot-cpu property from all DTS files

The 'linux,boot-cpu' property is obsolete, so remove it from all of the DTS
files and from booting-without-of.txt.  The boot CPU is actually defined in
the device tree header, and U-Boot sets that field.  The device tree compiler
also complains if the property exists.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
Acked-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>