mtd: spi-nor: spansion: prepare octal dtr methods for multi chip support

Infineon's multi-chip package (MCP) devices require the octal DTR
configuration to be set for each die. Split common code in
dedicated methods to ease the octal DDR MCP support addition.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Link: https://lore.kernel.org/r/20230726075257.12985-4-tudor.ambarus@linaro.org


Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>