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: Nkernel test robot <lkp@intel.com>
Signed-off-by: NRandy 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: NSerge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20201116040721.8001-1-rdunlap@infradead.orgSigned-off-by: NMark Brown <broonie@kernel.org>

drivers/spi/Kconfig

@@ -255,6 +255,7 @@ config SPI_DW_MMIO
 config SPI_DW_BT1
 	tristate "Baikal-T1 SPI driver for DW SPI core"
 	depends on MIPS_BAIKAL_T1 || COMPILE_TEST
+	select MULTIPLEXER
 	help
 	  Baikal-T1 SoC is equipped with three DW APB SSI-based MMIO SPI
 	  controllers. Two of them are pretty much normal: with IRQ, DMA,