spi: dw_bt1: fix MUX_MMIO dependencies

Selecting a symbol with additional dependencies requires
adding the same dependency here:

WARNING: unmet direct dependencies detected for MUX_MMIO
  Depends on [n]: MULTIPLEXER [=y] && OF [=n]
  Selected by [y]:
  - SPI_DW_BT1 [=y] && SPI [=y] && SPI_MASTER [=y] && SPI_DESIGNWARE [=y] && (MIPS_BAIKAL_T1 || COMPILE_TEST [=y])

Drop the 'select' here to avoid the problem. Anyone using
the dw-bt1 SPI driver should make sure they include the
mux driver as well now.

Fixes: 72188381 ("spi: dw-bt1: Fix undefined devm_mux_control_get symbol")
Fixes: abf00907 ("spi: dw: Add Baikal-T1 SPI Controller glue driver")
Link: https://lore.kernel.org/all/20221218192523.c6vnfo26ua6xqf26@mobilestation/Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20230130140156.3620863-1-arnd@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/Kconfig

@@ -295,7 +295,6 @@ config SPI_DW_BT1
 	tristate "Baikal-T1 SPI driver for DW SPI core"
 	depends on MIPS_BAIKAL_T1 || COMPILE_TEST
 	select MULTIPLEXER
-	select MUX_MMIO
 	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,