mtd: spi-nor: fsl-quadspi: fix unsupported cmd when run flash_erase

Erase function will use cmd 0x20 (SPINOR_OP_BE_4K) if kenrel enable option
CONFIG_MTD_SPI_NOR_USE_4K_SECTORS.

This command is not in fsl-quadspi driver LUT. So driver continue report
fsl-quadspi 21e0000.qspi: Unsupported cmd 0x20.

This patch fix this issue.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Acked-by: Han Xu <Han.xu@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>