ARM: imx: dh-imx6: Increase SF erase area for u-boot update

Erase the entire U-Boot area during U-Boot update instead of just
a subset of it. This way, in case u-boot-with-spl.imx grows, the
sf write won't write over non-erased part of the SPI NOR.

Signed-off-by: Marek Vasut <marex@denx.de>