make: Rename acorn_cle_215 to acorn and add acorn_pcie variant (with PCIe and UART crossover).
make: Rename acorn_cle_215 to acorn and add acorn_pcie variant (with PCIe and UART crossover).

Acorn PCIe can now be used with PCIe only with liteuart driver.

litex-hub/linux-on-litex-vexriscv,litex-hub/linux-on-litex-vexriscv