[POWERPC] remove include of asm/of_device.h from pmi.h

pmi.h does not diectly reference anything in of_device.h and of the two
files that include asm/pmi.h, one includes of_device.h and the other
includes of_platform.h (which includes of_device.h).

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>