drivers: mux: mmio-based syscon mux controller

This adds a driver for mmio-based syscon multiplexers controlled by
bitfields in a syscon register range.
This is heavily based on the linux mmio-mux driver.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
diff --git a/drivers/mux/Makefile b/drivers/mux/Makefile
index 351e436..78ebf04 100644
--- a/drivers/mux/Makefile
+++ b/drivers/mux/Makefile
@@ -4,3 +4,4 @@
 # Jean-Jacques Hiblot <jjhiblot@ti.com>
 
 obj-$(CONFIG_$(SPL_)MULTIPLEXER) += mux-uclass.o
+obj-$(CONFIG_$(SPL_)MUX_MMIO) += mmio.o