powerpc/configs: Disable SCOM_DEBUGFS in powernv_defconfig

SCOM_DEBUGFS is really not needed for anything other than low-level
hardware debugging.

mpe: It also introduces a large and poorly documented/understood
attack surface. Although the interface is only available to root, the
kernel still aspires to restrict root to accessing hardware through
well defined interfaces, which this is not.

opal-prd uses its own interface (/dev/prd) for SCOM access, so it
doesn't need SCOM_DEBUGFS.

At some point in the future we'll introduce a debug config fragment
where this can go instead.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190509051119.7694-5-ajd@linux.ibm.com