i.MX6: nitrogen6x: fix erase size in 6x_upgrade.txt

The 6x_upgrade script is used to upgrade U-Boot in SPI-NOR
on Nitrogen6x/SABRE Lite boards using U-Boot's 'sf' command.

U-Boot is placed at offset 0x400 in flash, and the script
currently only erases 0x50000 bytes. Since the current
head is 319k, any additional features enabled in the
configuration will exceed the space erased and cause errors
re-programming the device.

This patch increases the erase size to the full size of
the region allocated for the U-Boot binary.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>