spi: spi_bfin: handle multiple spi_masters

Move global SPI regs_base and dma_ch to struct driver_data.  Test on BF54x SPI
Flash with 2 spi_master devices enabled.
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>