[POWERPC] Rename MPIC_BROKEN_U3 to MPIC_U3_HT_IRQS

Rename MPIC_BROKEN_U3 to something a little more descriptive. Its
effect is to enable support for HT irqs behind the PCI-X/HT bridge on
U3/U4 (aka. CPC9x5) parts.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>