spi: zynq_qspi: Add function description

Add function description for zynq_qspi_init_hw and zynq_qspi_chipselect.
Fix zqspi to priv in function descriptions.
Change the description of priv as pointer to zynq_qspi_priv structure.
Fix other function descriptions to kernel-doc style.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>