Blackfin: bf537-stamp: drop board reset workaround

The bf537-stamp shouldn't need this SPI flash workaround.  It was added
by accident a long time ago through a convoluted series of steps which
originated from a customer board (not the bf537-stamp).  So drop it to
keep people from incorrectly adding it to their own boards.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>