spi: spi-xilinx: Added workaround when startup block is enabled

spi: spi-xilinx: Added workaround when startup block is enabled

In AXI SPI when the startup block is enabled in the hardware
STARTUP block don't provide clock as soon as SPI provides command,
first command fails. So to overcome this issue added a workaround
to perform a dummy Read Id if startup block is enabled in the
hardware.

Signed-off-by: Sai Krishna Potthuri <lakshmi.sai.krishna.potthuri@xilinx.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Acked-by: Naga Sureshkumar Relli <naga.sureshkumar.relli@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>