mtd: spi-nor: Add support for Spansion S25FL256L

mtd: spi-nor: Add support for Spansion S25FL256L

The S25FL256L is a part of the S25FL-L family and has the same feature set
as S25FL128L except the density.

The datasheet can be found in the following link.
https://www.cypress.com/file/316171/download

The S25FL256L is 32MB NOR Flash that does not support Bank Address
Register. This fixup is activated if CONFIG_SPI_FLASH_BAR is enabled and
returns ENOTSUPP in setup() hook to avoid further ops.

Tested on Xilinx Zynq-7000 FPGA board.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>