powerpc: Remove asm/prom.h from asm/mpc52xx.h and asm/pci.h

asm/pci.h and asm/mpc52xx.h don't need asm/prom.h

Declare struct device_node locally to avoid including of.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Add missing include of prom.h to of_rtc.c]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/cf5243343e2364c2b40f22ee5ad9a6e2453d1121.1657264228.git.christophe.leroy@csgroup.eu