mtd: spi: spi-nor-core: Add back U-Boot specific features

For legacy reasons, we will have to keep around U-Boot specific
SPI_FLASH_BAR and SPI_TX_BYTE. Add them back to the new framework

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> #zynq-microzed