ARM: imx6: dh-imx6: Use sf update instead of sf erase and write

With sf update fixed to support unaligned start offset, use plain
sf update to update the bootloader in SPI NOR. This also helps
avoid the case where not enough SPI NOR has been erased and the
bootloader has been written to unerased area, and thus corrupted.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>