Add build options to achieve 4 UARTs capes

• Add PCIE_ONLY buildoption for M.2 interface. This frees up the UART included in the M.2 DEFAULT build option to be re-assigned to the cape.
• Correct the M.2 PCIe block class code to report as root-complex.
• Add 4_UARTS build option. This brings four MSS UARTs to the cape connectors. Use this cape option with the PCIE_ONLY M2 build option in order to bring four UART to the cape connectors.