mtd: spi-nor: Create a local SR cache

mtd: spi-nor: Create a local SR cache

In order to be able to generate debugfs output without having to
actually reach the flash, create a SPI NOR local cache of the status
registers. What matters in our case are all the bits related to sector
locking. As such, in order to make it clear that this cache is not
intended to be used anywhere else, we zero the irrelevant bits.

The cache is initialized once during the early init, and then maintained
every time the write protection scheme is updated.

Suggested-by: Michael Walle <mwalle@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>