fpga: xilinx-selectmap: control CSI_B and RDWR_B during configuration

The driver requests the optional CSI_B and RDWR_B GPIOs with
GPIOD_OUT_HIGH and never touches them again. That flag carries a
logical value, so both signals end up asserted from probe on, whatever
polarity the device tree states, and they stay asserted for the
lifetime of the device.

Neither signal belongs to the driver's lifetime. CSI_B (Chip Select)
selects the device on the SelectMAP port, RDWR_B (Read/Write) selects
the transfer direction, so both belong to the data transfer. A device
that is never deselected never lets go of the port, and a port pinned
to write mode cannot be read back.

Keep the two descriptors in the driver private data, request them
deasserted, and assert them only around the configuration data
transfer. RDWR_B is asserted first, UG470 describes in chapter
"SelectMAP ABORT" that changing it while the device is selected aborts
the configuration on the next CCLK.

Signed-off-by: Heiko Schocher <hs@nabladev.com>
Reviewed-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20260909112200.13256-1-hs@nabladev.com
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>

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