powerpc/pseries: Don't override CONFIG_PPC_PSERIES_DEBUG

EEH and pci_dlpar #undef DEBUG, but I think they were added before the
ability to control this from Kconfig. It's really annoying to only get
some of the debug messages from these files. Leave the lpar.c #undef
alone as it produces so much output as to make the kernel unusable.
Update the Kconfig text to indicate this particular quirk :)

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Acked-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>