mtd: spi-nor: gigadevice: gd25q256: Init flash based on SFDP

Get rid of the static initialization of the flash parameters and
init them when parsing SFDP.
Generated a 256 Kbyte random data and did an erase, write, read back
and compare test. The flash uses for reads SPINOR_OP_READ_1_4_4_4B 0xec,
for erases SPINOR_OP_BE_4K_4B 0x21, and for writes SPINOR_OP_PP_1_1_4_4B
0x34.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20211207140254.87681-14-tudor.ambarus@microchip.com