boards: litex: move changeable peripherals to board

move changeable peripherals from dtsi to board, as
in litex these can change and might be on
different registers on custom out of tree
boards. So we limit riscv32-litex-vexriscv.dtsi
to just the interrupt controller and the cpu,
which don't change.

Signed-off-by: Fin Maaß <[email protected]>

Author: maass-hamburg