mtd: spi-nor: eliminate duplicate spi_nor_wait_till_{, fsr}_ready() code

These functions were near-carbon-copies due to a small per-flash quirk.
Let's add a new spi_nor::flags bitfield to support these types of
quirks.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Cc: Graham Moore <grmoore@altera.com>
Cc: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>