powerpc/powernv: Move SCOM access code into powernv platform

The powernv platform is the only one that directly accesses SCOMs.
Move the support code to platforms/powernv, and get rid of the
PPC_SCOM Kconfig option, as SCOM support is always selected when
compiling for powernv.

This also means that the Kconfig item for CONFIG_SCOM_DEBUGFS will
show up in menuconfig in the platform menu, rather than at the root,
which is a much better location.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190509051119.7694-1-ajd@linux.ibm.com

arch/powerpc/platforms/powernv/opal-xscom.c

@@ -14,7 +14,8 @@
 #include <asm/machdep.h>
 #include <asm/firmware.h>
 #include <asm/opal.h>
-#include <asm/scom.h>
+
+#include "scom.h"
 
 /*
  * We could probably fit that inside the scom_map_t