mtd: rawnand: constify elements of NAND_OP_PARSER(_PATTERN)

Currently, drivers are able to constify a nand_op_parser array,
but not nand_op_parser_pattern and nand_op_parser_pattern_elem
since they are instantiated by using the NAND_OP_PARSER(_PATTERN).

Add 'const' to them in order to move more driver data from .data to
.rodata section.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>