mtd: spinand: Use more specific naming for the page read op

SPI operations have been initially described through macros implicitly
implying the use of a single SPI SDR bus. Macros for supporting dual and
quad I/O transfers have been added on top, generally inspired by vendor
naming, followed by DTR operations. Soon we might see octal
and even octal DTR operations as well (including the opcode byte).

Let's clarify what the macro really means by describing the expected bus
topology in the page read macro name.

Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>