spi: xilinx_spi: Modify transfer logic xilinx_spi_xfer() function

This patch modify xilinx_spi_xfer() function and add rxfifo() and
txfifo() functions to add the modularity so that these functions
can be used by other functions within the same file.

This patch also added support to read fifo_size from dts.

Signed-off-by: Vipul Kumar <vipul.kumar@xilinx.com>
Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>