Blackfin: bf533-stamp/bf537-stamp: fix env settings for SPI flash

The SPI flash layer is much stricter about sector usage than the eeprom
layer we used to use, so update the env settings to better match the
sector alignment of the flashes we use.

Signed-off-by: Vivi Li <vivi.li@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>