spi: xilinx_spi: Use BIT macro

Replace numerical bit shift with BIT macro
in xilinx_spi

:%s/(1 << nr)/BIT(nr)/g
where nr = 0, 1, 2 .... 31

Cc: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>