spi: dw: Use generic function to read reg address

Using an fdt-specific function causes problems when compiled with a live
tree.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Tested-by Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>