mtd: spi-nor: fix kernel-doc for spi_nor::spimem

When adding the 'spimem' field to 'struct spi_nor', a grammar mistake
("point" instead of "pointer") was made -- fix it and convert the SPI
acronym to uppercase and fully spell out "memory", while at it...

Fixes: b35b9a10 ("mtd: spi-nor: Move m25p80 code in spi-nor.c")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>