wl18xx: add board type module argument

Different board types (ie. FPGA, HDK and DVP/EVB) require slightly
different init configuration options.  Since we cannot probe the type
of board from the actual hardware, we need to pass it as an option
during module load.

This patch adds a module parameters that accepts the 3 different board
types, with DVP/EVB as the default, and uses this value where needed.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>