FPGA: add support for downloading Lattice bitstream

The patch adds support to load a Lattice's bitstream
image (called VME file) into a Lattice FPGA. The code
containing the state machine delivered as part of
Lattice's ispVMtools is integrated.

The FPGA is programmed using the JTAG interface. The
board maintainer must provide accessors to drive the
JTAG signals TCK, TMS, TDI and to get the value of the
input signal TDO.

Signed-off-by: Stefano Babic <sbabic@denx.de>