mux: mmio: Fix missing CONFIG_REGMAP_MMIO

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 <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505150312.dYbBqUhG-lkp@intel.com/
Fixes: 61de83fd8256 ("mux: mmio: Do not use syscon helper to build regmap")
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20250521152354.92720-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250616093841.31985-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>