mux: mmio: Fix missing CONFIG_REGMAP_MMIO

MMIO mux uses now regmap_init_mmio(), so one way or another
CONFIG_REGMAP_MMIO should be enabled, because there are no stubs for
!REGMAP_MMIO case:

  ERROR: modpost: "__regmap_init_mmio_clk" [drivers/mux/mux-mmio.ko] undefined!

Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Fixes: 61de83f ("mux: mmio: Do not use syscon helper to build regmap")
Acked-by: Arnd Bergmann <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Krzysztof Kozlowski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: krzk

drivers/mux/Kconfig

@@ -48,6 +48,7 @@ config MUX_GPIO
 config MUX_MMIO
 	tristate "MMIO/Regmap register bitfield-controlled Multiplexer"
 	depends on OF
+	select REGMAP_MMIO
 	help
 	  MMIO/Regmap register bitfield-controlled Multiplexer controller.