spi: Remove dual flash options/flags

Dual flash code in spi are usually take the spi controller
to work with dual connected flash devices. Usually these
dual connection operation's are referred to flash controller
protocol rather with spi controller protocol, these are still
present in flash side for the usage of spi-nor controllers.

So, this patch remove the dual_flash options or flags in sf
which are triggered from spi controller side.

Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: York Sun <york.sun@nxp.com>
Cc: Vignesh R <vigneshr@ti.com>
Cc: Mugunthan V N <mugunthanvnm@ti.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>