spi: spi_bfin uses platform device resources

Update spi driver to support multi-ports by using platform resources; tested
on STAMP537+SPI_MMC, other boards need more testing.  Plus other minor
updates.

Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>