QCA955x / AP135 platform tidyups.

• Change mips24k -> mips74k for hwpmc, but leave it disabled for now.
• don't build pci by default.
• build pci and qca955x_pci for AP135, as theres a PCIe NIC.
• don't build a hwpmc module, it doesn't really work out well for the mips boards at the moment.
• add ipfw and DEFAULT_TO_ACCEPT.