powerpc: Fix config dependency problem with MPIC_U3_HT_IRQS

MPIC_U3_HT_IRQS is selected both by PPC_PMAC64 and PPC_MAPLE, but depends
on PPC_MAPLE, so a PPC_PMAC64-only config gets this warning:

warning: (PPC_PMAC64 && PPC_PMAC && POWER4 || PPC_MAPLE && PPC64 && PPC_BOOK3S) selects MPIC_U3_HT_IRQS which has unmet direct dependencies (PPC_MAPLE)

Fix that by removing the dependency on PPC_MAPLE.

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>