mtd: spi-nor: re-name OPCODE_* to SPINOR_OP_*

Qualify these with a better namespace, and prepare them for use in more
drivers.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Huang Shijie <b32955@freescale.com>