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: NBryan Wu <bryan.wu@analog.com>
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>