powerpc/85xx: Remove DBG() macro

DBG() macro is defined at three places while used only
one time at one place.

Replace its only use by a pr_debug() and remove the macro.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230408140122.25293-2-pali@kernel.org