powerpc: Finally remove unnecessary headers from asm/prom.h

Remove all headers included from asm/prom.h which are not used
by asm/prom.h itself.

Declare struct device_node and struct property locally to
avoid including of.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4be954abef978b34cff9193fc566ffefdd3517bb.1657264228.git.christophe.leroy@csgroup.eu