[POWERPC] Get most of ioctl.h content from <asm-generic/ioctl.h>

Now that <asm-generic/ioctl.h> allows overriding of the most commonly
changed macros, take advantage of that.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>