spi: dw: fix build error by selecting MULTIPLEXER

Fix build error for spi-dw-bt1.o by selecting MULTIPLEXER.

hppa-linux-ld: drivers/spi/spi-dw-bt1.o: in function `dw_spi_bt1_sys_init':
(.text+0x1ac): undefined reference to `devm_mux_control_get'

Fixes: abf00907 ("spi: dw: Add Baikal-T1 SPI Controller glue driver")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-spi@vger.kernel.org
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20201116040721.8001-1-rdunlap@infradead.org


Signed-off-by: Mark Brown <broonie@kernel.org>