mtd: rawnand: omap2: Fix build warnings with W=1

mtd: rawnand: omap2: Fix build warnings with W=1

Add kernel-doc for functions to get rid of below warnings
when built with W=1.

drivers/mtd/nand/raw/omap2.c:260: warning: Function parameter or struct member 'chip' not described in 'omap_nand_data_in_pref'
drivers/mtd/nand/raw/omap2.c:260: warning: Function parameter or struct member 'buf' not described in 'omap_nand_data_in_pref'
drivers/mtd/nand/raw/omap2.c:260: warning: Function parameter or struct member 'len' not described in 'omap_nand_data_in_pref'
drivers/mtd/nand/raw/omap2.c:260: warning: Function parameter or struct member 'force_8bit' not described in 'omap_nand_data_in_pref'
drivers/mtd/nand/raw/omap2.c:304: warning: Function parameter or struct member 'chip' not described in 'omap_nand_data_out_pref'
drivers/mtd/nand/raw/omap2.c:304: warning: Function parameter or struct member 'buf' not described in 'omap_nand_data_out_pref'
drivers/mtd/nand/raw/omap2.c:304: warning: Function parameter or struct member 'len' not described in 'omap_nand_data_out_pref'
drivers/mtd/nand/raw/omap2.c:304: warning: Function parameter or struct member 'force_8bit' not described in 'omap_nand_data_out_pref'
drivers/mtd/nand/raw/omap2.c:446: warning: Function parameter or struct member 'chip' not described in 'omap_nand_data_in_dma_pref'
drivers/mtd/nand/raw/omap2.c:446: warning: Function parameter or struct member 'buf' not described in 'omap_nand_data_in_dma_pref'
drivers/mtd/nand/raw/omap2.c:446: warning: Function parameter or struct member 'len' not described in 'omap_nand_data_in_dma_pref'
drivers/mtd/nand/raw/omap2.c:446: warning: Function parameter or struct member 'force_8bit' not described in 'omap_nand_data_in_dma_pref'
drivers/mtd/nand/raw/omap2.c:467: warning: Function parameter or struct member 'chip' not described in 'omap_nand_data_out_dma_pref'
drivers/mtd/nand/raw/omap2.c:467: warning: Function parameter or struct member 'buf' not described in 'omap_nand_data_out_dma_pref'
drivers/mtd/nand/raw/omap2.c:467: warning: Function parameter or struct member 'len' not described in 'omap_nand_data_out_dma_pref'
drivers/mtd/nand/raw/omap2.c:467: warning: Function parameter or struct member 'force_8bit' not described in 'omap_nand_data_out_dma_pref'

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412031716.JfNIh1Uu-lkp@intel.com/
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>